If it's done right: easier to do simple things with, faster development cycle, run time evaluation of code... all the usual reasons why people use interpreted languages instead of compiled ones.
I take it you've never used Objective-C and Cocoa.
Objective-C is a very dynamic language, especially among compiled languages. It was based on Smalltalk, the same language Python and Ruby were influenced by.
There is really very little room for JavaScript to improve upon Objective-C.
Javascript is simpler - but more importantly, much more popular than Objective-C. It's also garbage collected, a feature that Objective-C may also have these days, but not when I last used it a couple of years ago.
Javascript's also a hell of a lot more popular than ruby, haskel, lisp, <insert your favorite dynamic language here>, etc. But nobody here is saying "Let's give up ruby and switch to Javascript!" Popularity is not everything.
(It's also worth noting that Objective-C itself may well be as popular, or more so, than most of those same languages. It is, of course, a difficult thing to measure, but there is some evidence to suggest its at least more popular than languages like Lisp)
