While you can use ClojureScript (CLJS) server-side running on say node.js, the largest benefit is as a replacement for client-side JS (ClojureScript converts to JS at compile-time).

You can actually use all the libraries you mentioned (jQuery, underscore) from ClojureScript. What CLJS really brings to the table are a set of abstractions and data structures that make it easier to write strong reusable functional code. In fact, alot of underscore comes built into CLJS and I think much of what jQuery buys you can be handled as a good CLJS library (but more on that soon!)