> Can someone explain to me what one could do with this?

Just to give this a quick and incomplete answer:

- you could rewrite the tree before executing it: 
http://rewrite.rubyforge.org/

- you could transform ruby into sql: 
http://errtheblog.com/posts/63-full-of-ambition

- you could transform ruby into ruby (think serialisation): 
http://seattlerb.rubyforge.org/ruby2ruby/

- or you could write a ruby vm on top of ruby (shameless plug!): 
https://github.com/kschiess/verneuil

& much much more! For inspiration, read on Lisp and transpose to Ruby ;)

kaspar