Well, have you ever tried to program in javascript? It's a fantastic language for prototyping. If you want to try an algorithm, you sit down, write an astonishly small quantity of code and it just works. That's just one of the reason why I use javascript.

Another is, that it's a fantastic language to use for hobby projects, because you don't need to think too much beforehand, thanks to the dynamically typed variables, methods and objects.

And last but not least, you don't have any GUI to manage. HTML does it for you, which simplifies matters a bit. You want to dynamically create HTML? No problem. You prefer to prepare the HTML beforehand? No problem at all.

So to answer your question, I use it because it's refreshingly easy to use, more so than richer languages who are sometimes simply too heavy.