I don't usually answer this type of question, but for some reason I'm in the mood today.
A fast 50mm lens on a DX body fits into the low end of what most photographers would call the "portrait lens" range. In other words, you can isolate the upper body and face and throw the background out of focus with a fast aperture, giving you the classic "it's just about the person" shot.
But here's the reason why I don't usually answer such questions: that use of a fast 50mm on DX results in a photographic cliche. And it's not the only thing you could use a 50mm lens for.
The real answer is that photographers have a range of choices in their camera bag. The bigger the range of choices, the more things they can do, whether it be cliche, or perhaps something less cliche such as using the 50mm to do a huge landscape pano stitching project (e.g. lots of semi-telephoto shots with detail stitched into a wide angle landscape shot).
