Different interpretations of the 6 realms What are the psychological and metaphorical interpretations of the 6 realms and why are they significant to some Buddhists but not others?
The psychological reality of the 6 realms is real &amp; not metaphorical. Where as the  interpretations of the 6 realms as places that exist elsewhere, after the ending of life, is superstition. For example, when a vicious rapist or murderer or psychopath politician in the world is called "an animal", this is real &amp; not a metaphor. They are animals &amp; they are not human. In Buddhism, the word "human" ("manussa") means "high-minded" or "lofty-minded". The Pali suttas state a "human" lives in accordance with the Dhamma; with righteous conduct; without violence or exploitation ("devouring") of other beings. From the Buddhist point of view, not all people are "human". The book Buddha-Dhamma for Students may help here.

The real right understanding of the six realms is important to some Buddhists so they can see the world clearly (with dispassion), so they live the right way &amp; so they can attain enlightenment. In other words, so they do not overestimate their spiritual progress &amp; state. For example, many people who believe they are Buddhists &amp; who are addicted to entertainment, sensuality, pornography, luxury, etc, believe the mind is "human" rather than the reality the mind is "hungry ghost". They believe by having superficial faith in the Buddha they will have a good reincarnation &amp; don't realise the mind, in the here-&amp;-now, is hungry ghost. This is how they overestimate their spiritual state. 

Some Pali suttas listed below, which show the realms are mental states in the here-&amp;-now. 


  I have seen, bhikkhus, the hell named ‘Contact’s Sixfold Base.’ There
  whatever form one sees with the eye is undesirable, never desirable;
  unlovely, never lovely; disagreeable, never agreeable. Whatever sound
  one hears with the ear … Whatever odour one smells with the nose …
  Whatever taste one savours with the tongue … Whatever tactile object
  one feels with the body … Whatever mental phenomenon one cognizes with
  the mind is undesirable, never desirable; unlovely, never lovely;
  disagreeable, never agreeable.
  
  I have seen, bhikkhus, the heaven named ‘Contact’s Sixfold Base.’
  There whatever form one sees with the eye is desirable, never
  undesirable; lovely, never unlovely; agreeable, never disagreeable.
  Whatever sound one hears with the ear … Whatever odour one smells with
  the nose … Whatever taste one savours with the tongue … Whatever
  tactile object one feels with the body … Whatever mental phenomenon
  one cognizes with the mind is desirable, never undesirable; lovely,
  never unlovely; agreeable, never disagreeable.
  
  SN 35.135
  
  
  
  How has a monk attained/arrived at the state of a god?... A monk
  enters &amp; dwells is the first...second...third...fourth jhana...
  
  How has a monk attained/arrived at the state of a brahma?... A monk
  pervades the entire world with loving-kindness... compassion...
  appreciative joy... equinimity... 
  
  AN 4.190
  
  
  
  Living with Brahma are those families where, within the home, mother
  and father are respected by their children. Living with the early
  devas (gods) are those families where, within the home, mother and
  father are respected by their children. For what reason? Because
  mother and father are very helpful to their children, they take care
  of them and bring them up and teach them about the world.
  
  Mother and father are called "Brahma," "early teachers" And "worthy of
  veneration," Being compassionate towards Their family of children.
  
  Iti 109
  
  
  
  Bhikkhus, a god, a human or any other good state would not be evident
  from actions born of greed, hate and delusion. Yet, bhikkhus, from
  actions born of greed, hate and delusion a hellish being, an animal
  birth a ghostly birth or some other bad state would be evident.
  
  AN 6.39
  
  
  
  Bhikkhus, suppose a man would throw a yoke with a single hole into the
  great ocean, and there was a blind turtle which would come to the
  surface once every hundred years. What do you think, bhikkhus, would
  that blind turtle, coming to the surface once every hundred years,
  insert its neck into that yoke with a single hole?
  
  If it would ever do so, venerable sir, it would be only after a very
  long time.
  
  Sooner, I say, would that blind turtle, coming to the surface once
  every hundred years, insert its neck into that yoke with a single hole
  than the fool who has gone once to the nether world would regain the
  human state. For what reason? Because here, bhikkhus, there is no
  conduct guided by the Dhamma, no righteous conduct, no wholesome
  activity, no meritorious activity. Here there prevails mutual
  devouring, the devouring of the weak. For what reason? Because,
  bhikkhus, they have not seen the Four Noble Truths. What four? The
  noble truth of suffering … the noble truth of the way leading to the
  cessation of suffering.
  
  SN 56.47
  
  
  
  Now on that occasion the wanderers of other persuasions had come
  together in a gathering and were sitting, discussing many kinds of
  bestial (animal) topics, making a great noise and racket. AN 10.93
  
  
  
  Bhikkhus, these two bright principles protect the world. What are the
  two? Shame and fear of wrongdoing. If, bhikkhus, these two bright
  principles did not protect the world, there would not be discerned
  respect for mother or maternal aunt or maternal uncle’s wife or a
  teacher’s wife or the wives of other honored persons, and the world
  would have fallen into promiscuity, as with goats, sheep, chickens,
  pigs, dogs, and jackals. But as these two bright principles protect
  the world, there is discerned respect for mother… and the wives of
  other honored persons. AN 2.9
  
  
  
  Beings (sattā) are few who, when they pass away from the animal realm
  (tiracchānayoniyā) are 'reborn' (paccājāyanti) among humans
  (manussesu)… those beings are more numerous are reborn in hell…. For
  what reason? They have not realised the Four Noble Truths. SN 56.102
  to 131
  
  
  
  Householders, there are these four ways of living together. What four?
  
  A wretch lives together with a wretch; a wretch lives together with a
  female deva (god); a deva (god) lives together with a wretch; a deva
  (god) lives together with a female deva (god).
  
  When both are immoral, miserly and abusive, husband and wife live
  together as wretches.
  
  The husband is immoral, miserly and abusive, but his wife is virtuous,
  charitable, generous. She is a female deva living with a wretched
  husband.
  
  The husband is virtuous, charitable, generous, but his wife is
  immoral, miserly and abusive. She is a wretch living with a deva
  husband.
  
  Both husband and wife are endowed with faith, charitable and
  self-controlled, living their lives righteously, addressing each other
  with pleasant words.
  
  Then many benefits accrue to them and they dwell at ease. Their
  enemies are saddened when both are the same in virtue.
  
  Having practiced the Dhamma here, the same in virtuous behavior and
  observances, delighting in a deva world, they rejoice, enjoying
  sensual pleasures.
  
  AN 4.53
  
  
  
  And what is the origination of the world? Dependent on the eye &amp; forms
  there arises eye-consciousness. The meeting of the three is contact.
  From contact as a requisite condition comes feeling. From feeling as a
  requisite condition comes craving. From craving as a requisite
  condition comes clinging. From clinging as a requisite condition comes
  becoming. From becoming as a requisite condition comes birth. From
  birth as a requisite condition, then aging &amp; death, sorrow,
  lamentation, pain, distress, &amp; despair come into play. This is the
  origination of the world. SN 12.44
  
  
  
  Here, Udayi, the bhikkhu secluded from sensual desires and thoughts of
  demerit abides in the first jhana: Overcoming thoughts and thought
  processes and the mind in one point internally appeased, without
  thoughts and thought processes abides in the second jhana. Again with
  equanimity to joy and detachment, feeling pleasant with the body too,
  abides in the third jhana. To this the noble ones say abiding in
  pleasantness with equanimity. Udayi, this is the course of actions,
  for realising the world of only pleasant feelings (ekantasukhassa
  lokassa). MN 79
  
  
  
  Sensual desire… ill-will (anger)… sloth &amp; drowsiness… restlessness &amp;
  anxiety… uncertainty is an obstacle, a hindrance that overwhelms the
  mind and weakens wisdom… when a monk has not abandoned these five
  obstacles… for him to understand what is for his own benefit, to
  understand what is for the benefit of others, to understand what is
  for the benefit of both, to realize a superior human (manussa) state
  (dhammā), a truly noble distinction in knowledge &amp; vision: that is
  impossible. AN 5.51