50mm is the so-called "normal" lens. You might choose a wide or a long lens for either the look it gives you, or for practical reasons. Or you might choose a 50mm because it does NOT give you a certain look, or because it is the only focal length that can work, given the subject and the space you have to work in.
If you are using a fast 50mm lens, like a 1.8, 1.4 or 1.2 the speed of the lens is usually the main reason to use it — for the way it throws the background out of focus.
Bottom Line: Use a 50mm lens because you want to avoid the more stylized look of wider and longer lenses, or because it is the only practical solution.
When you shoot with a general-purpose zoom, you may zoom to roughly 50mm without thinking much about focal length.
This summary of 50mm as a focal length suggests that it is rarely a chosen lens (other than a high-speed 50) — that it is generally a compromise lens. I asked a full-time pro friend about this, and he said he hasn't had a 50 on his camera in 30 years. A survey sample of one is not accurate, but you get the idea.