honestly, I don't even know how to answer that question, but I'll try:
It's like asking "Why should I bother learning German if I already know English?" Maybe you want to talk to people in Germany, Austria and Switzerland? Maybe you want to order a meal at a restaurant there? Maybe you want to move there or open a business?
Just as learning new spoken languages is either useful or necessary, depending on what you want to or have to do, so is learning new programming languages.
You want to use Ruby on Rails? Well, you'll need to learn Ruby because Rails happens to be written in Ruby.
You want to develop for the Mac or iOS? Well, you'll need to learn Objective-C because all of the Cocoa frameworks are written in Obj-C (or C).