How are the 31 planes of existence (especially Bṛhatphala planes) a practical metaphor? This comment suggests that the various lokas and beings inhabiting them can be taken metaphorically, i.e. it said,

Many of these can be answered with references back to metaphorical interpretations, by both traditional lineages and by the modern secular Buddhism movement. If I were you I would ask questions about the meaning behind the metaphorical language and how it can be applied to solve the problems of life and death.


So, as a follow-on from that comment — i.e. if or when they are interpreted metaphorically:

Is there an overarching metaphorical meaning of the 31 lokas, and the various kinds of beings that inhabit them? What is the meaning, and how does this meaning help the practitioners of Buddhism, and all earthly beings?

Wikipedia descrides the Bṛhatphala planes as follows — unless this reference is inaccurate, would you explain how its metaphorical meaning can help practitioners and all earthly beings?



Bṛhatphala Planes
These two realms are a result of attaining the fourth jhana. They remain in the tranquil state attained in the 4th Jhana, and is characterized by equanimity (upekṣā).

22 - Unconscious beings (Asaññasatta) Realm of mindless beings who have only bodies without consciousness. Rebirth into this plane results from a meditative practice aimed at the suppression of consciousness. Those who take up this practice assume release from suffering can be achieved by attaining unconsciousness. However, when the life span in this realm ends, the beings pass away and are born in other planes where consciousness returns.

21 - Very Fruitful devas (vehapphala deva) In the Jhana Sutta of the Anguttara Nikaya the Buddha said &quot;The Vehapphala devas, monks, have a life-span of 500 eons. A run-of-the-mill person having stayed there, having used up all the life-span of those devas, goes to hell, to the animal womb, to the state of the hungry shades.&quot;
Is there an overarching metaphorical meaning of the 31 lokas, ...


According to the metaphorical style of interpretation, 31 planes of existence are a map of various lifestyles and mind states (with lower realms corresponding to neurotic conditions, and the higher realms to the meditative trance states) with examples of attitudes leading to and perpetuating such states.


  ... and the various kinds of beings that inhabit them? What is the meaning, ...


Based on my study of texts and my insight meditation, here are some very condensed and simplified descriptions. Please don't take these as all-encompassing definitions, they are just basic pointers:


hell is a subjective state characterized by heavy antipathy and helplessness, in which "how things are" is 100% opposite of "how you think it should be" and you can't do anything about it.
animal realm is a life style characterized by complacency towards hardships and inconveniences, ignorance, lowly values and lack of ownership over one's destiny.
preta's realm is a neurotic condition in which confusion, craving and negative prejudice continuously give rise to a cycle of false hope to attain something 100% worthy, alternating with bitter disappointment when every consecutive object of one's pursuit fails to satisfy one's childishly simplified expectations.
human's realm is lifestyle and condition dominated by attachment to and preoccupation with what one holds important or dear, such as errands, projects, group interests, conflicts etc.
asura's realm is a mindset and attitude of extreme intelligence and ambition / competitiveness, without regards for others' well-being.
gods' realm or heaven is a temporary state when the objective state of things 100% matches one's expectations and one is deluded into assuming that things will never change. Deva' realms from 6 to 11 are different kinds of this state.
the form realms (#12 to 27) are semi-enlightened lifestyles and mindstates characterized by predominance of abstract thinking and substantial levels of insight into the nature of phenomena, resulting in relative absence of strong emotions. The only remaining source of conflicts and suffering in these realms is rigid adherence to one or the other philosophic position.
the formless realms (#28 to 31) are the predominant mindstates of yogis bent on the practice of transcendental absorption (cultivation of meditative trance) which are characterized by a temporary bliss followed by the inevitable return to the empirical world with its problems and pains.



  ... and how does this meaning help the practitioners of Buddhism, ...


Study and meditation on the realms helps practitioners of Buddhism to disidentify / step aside from their own biases and states of mind, as well as to identify the conditions of others, and hopefully pick an appropriate corrective action towards the Liberation-By-Wisdom.


  ... how [realms #21 and 22] metaphorical meaning helps practitioners ...


Realms #21 and 22 is a reference to mindstates of non-buddhist practitioners succeeding in meditation. Even though they may achieve bliss (#21) or suspension of consciousness (#22) because of their Wrong View they are bound to fall back to the lower realms. Metaphorical illustrations like these help the practitioners of Buddhism understand the subtle differences between correct and incorrect practices at advanced stages of the path.


  ... and all earthly beings?


All Buddhist practices directly or indirectly help "all earthly beings" as you call them, by reducing the amount of "suffering pollution" generated due to confusion and ignorance.