The two big things are the ability to reason about your logic with a consistent mechanism (asynchronous streams) as well as the very rich set of supported operators for those streams. This allows you to treat asynchronous inputs (AJAX callbacks, I/O, UI events, etc) similarly, which can make gluing the pieces together a lot nicer.

It’s used a lot more on the front end than on the back end, in my experience. But I really enjoy working with it.