The obvious answer is because it's cool.
Seriously though, tinkering with Common Lisp did prepare me a bit for my first Rails job since Ruby is, as I describe it, like "Perl and Smalltalk had a baby with Lisp sitting in the closet dressed like Superman, watching". Pragmatically, you might not need to touch Lisp, but going through it may show you some common pitfalls that occur in other languages and makes you wish there were Lispy features outside of it.