I have a heavy web dev background with PHP. My reasons for learning a functional programming languages are:
Also which has the better community support, tutorials and books and the better web application framework?
Feel free to suggest other languages.
Seriously, the more languages you have under your belt, the better off you'll be as a programmer regardless of the language you end up using.
Rant: "What language should I learn?" questions are getting tiresome. The answers should be the same every time, yet no one gives the correct sage advice. Instead, everyone spreads FUD and suggests that the language they happen to use for their day job is the one you should learn. Poppycock.