What is the purpose of chiasmus?
I think it has several connected purposes...
As a self confessed fan of chiasmus, I think it has several purposes:
To give structure and poetic form, making text more digestible and memorable.
To set the scene and draw the reader from where they are into the core, from the outer courts into the inner courts.
To protect the essential core at the heart of the chiasmus and draw attention to it.