Now, I know what you're thinking... If you've got 4 JS files for a total of 50KB then combined they're still going to take 50KB. Yeah, that's 100% true. You're probably also thinking, "why should I compress my pages when httpd can do that on the fly?" Well, yeah, that's also true.
The benefit here is that not only are the combined & compressed versions cached so the compression only occurs once per change (avoiding the CPU & memory usage from that task) but in combining the individual files into one, you're avoiding the overhead of having to open multiple connections to a server in order to download all of the files.
As I mentioned above, I tried this on a new site I'm developing and it cut the page load time down from 1.6-1.7 seconds to 0.8-0.9 seconds (according to the timer in the "FasterFox" plugin for Firefox) which is pretty substantial.