Why are there so many programming languages?

I'm pretty fluent in C/C++, and can make my way around the various scripting languages (awk/sed/perl). I've started using python a lot more because it combines some of the nifty aspects of C++ with the scripting capabilities of awk/sed/perl. But why are there so many different programming languages? I'm guessing all these languages can do the same things, so why not just stick to one language and use that for programming computers? In particular, is there any reason I should know a functional language as a computer programmer? Some related reading: Why new programming languages succeed -- or fail? is there still research to be done in programming languages?