The spiritual significance of the sand mandala in Tibetan Buddhism I have heard of and seen sand mandalas in Tibetan Buddhism.

Here is a picture:



I have two questions, which are related:


I have heard that upon completion they are erased to represent transience. Is this what the erasure represents?
What other spiritual significance does the mandala have in Tibetan Buddhism. I seem to remember something relating to a spiritual journey being represented in the mandala yet I am unsure. I would like clarification on this.
According to The Tibetan Monks of Drepung Gomang Monastery





  When finished, to symbolize the impermanence of all that exists, the colored sands are swept up and poured into a nearby river or stream where the waters carry healing energies throughout the world.
  
  
  
    In general, all mandalas have outer, inner, and secret meaning.  On the outer level they represent the world in its divine form; on the inner level, they represent a map by which the ordinary human mind is transformed into the enlightened mind; and on the secret level, they predict the primordially perfect balance of the subtle energies of the body and the clear light dimension of the mind.  The creation of a sand painting is said to affect purification and healing on these three levels.