MSC codes are a nice way to keep an eye on what's appearing in your area. Back in the "bad old day" before online MathSciNet and the ArXiv, I used to go to the library every month or two and skim all of the Math Reviews in 11D and 11G, and I reasonably frequently found articles of interest that I would then go read. Now I'm a bit lazy, but at least a few times a year I'll bring up all the 11G and 37P reviews and skim the titles, then look at the reviews of the ones that seem interesting. (Of course, these days often I already know about them from the ArXiv.) So I guess I'm echoing some of the other responses in saying that if you're researching a particular topic, searching on key words is much better, but if you want a general overview of what sorts of things people in your area are working on, it can be quite instructive to look at titles (and reviews) in an MSC category. BTW, there's a feature on MathSciNet that lets you bring up articles whose reviews have appeared in the past month(s). This is useful, since otherwise there will be a lot of articles that appear in the list that are waiting to be reviewed.