Let's face it: browsing the Android Market on your desktop currently sucks. For one thing, there aren't any categories - just "Top Free" and "Top Paid." For another, there's no search!
On your phone, the situation isn't much better - you can search, which is good if you know what you're looking for in advance but you still can't filter those results. And discovering new apps is just downright horrible.
This is where sites like AppBrain come in.