Because folks feel there is a need for them, and it's a really good way to get one's feet wet in the compiler pound. Besides, a programming language (dialect) is to a programmer what different size wenches, pipes, or hoes are to a plumber. You need different tools for different jobs and a lot of it depends on your personal preference and what it will take you to be efficient. Besides, how we as engineers solve problems is just as much an expression of our personality as it is a statement about the correct operation of systems. But that's just my view, and I'm definitely biased on the subject because I'm working on a flavor of coffeescript that will compile to msft's typescript as we speak (well, in my spare time).