Very simple: JS (module pattern, encapsulation) is verbose. CoffeeScript is more compact, allowing for more code on one page. I toy with going back to JS (I don't like everything about CS, especially with debugging and test tools in node development) but CS optimizes for the good parts of JS and ignores the parts I don't want to use anyway.

As a guy who spends a lot of time writing JS, what are the good parts of JS that CS optimizes for, and what are the parts it ignores that you do not want to use?

It significantly reduces the symbol noise of JS. Typical JS is a dense forest of nested "function" expressions with a ton of (){} and it takes a lot of effort to fish out the logic.

CS strips out a lot of that and makes the intention of the code clearer.