Simplicity vs. feature bloat
Clojure and ClojureScript are built on the idea of immutable data. You can build a new data object on an existing one, but you don't normally change one that already exists. Client-side programming depends heavily on asynchronous processes. The client sends a request to the server and waits for a result, but the user may do something in the meantime. Users may click buttons or resize the window at any time, causing a function to run.
Immutability reduces the chances for coding bugs in asynchronous processing. A variable won't mysteriously change because another thread is messing with it. Pages behave predictably.
Why the language matters
Clients ultimately don't care what language a Web page is written in; they just want one that does what it's supposed to. But a language that's clean, one that doesn't clutter the process with ten ways to do the same thing, lets developers focus their imagination on what they can do. It can let them see their way to functionality that they might not have considered otherwise. A language that encourages clean, consistent coding tends to have fewer bugs. Developers who take over a project later can understand it more easily.