The 50mm lens is one of the most used lenses in my bag, and many other photographer's bag as well. Why? Well, here are my reasons why I own and use one:
1. It came with my camera when I bought it. Ok, that might not sound like a reason but it is one. But, I did purchase a "better" one later. When I say "better" I really mean one that has different properties.
2. It is one of the most inexpensive "prime" (meaning not zoom) lens you can purchase. Most lens manufacturers make and sell a lot of 50mm lenses, so they can afford to sell them at a lower profit margin then other lenses. Canon sells their 50mm F1.8 for $125, f1.4 for $329, and their best lens; an F1.2 for $1,349. Its all about the bokeh! (and sharpness for some)
3. Size vs weight. The "nifty-fifty" as it is sometimes referred to, is a very small and light lens (although the F1.2 is relatively heavy). It is a perfect lens to keep in your camera. In fact Canon makes a 40mm lens, sometimes called a Pancake lens for reasons that never made sense to me, is almost as small as the protective lens cap that you put on the camera body when there is no lens attached, so why not keep it on there all the time? The 50mm is a perfect "walking-around" lens. Trying to walk around a crowded city or amusement park, or birthday party with a long 70-200mm lens can be challenging to say the least.
4. My 50mm F1.4 is "fast." Not like race car fast, but cat fast, as in they can see in the dark. Actually the cat's eye and the 50mm F1.4 and, even more so, the F1.2 all let in a lot of light, so working in low light situations, or in situations where a very fast shutter speed is required, this lens will be your best bet.
5. The lens best represents what the human eye sees. By that I mean, the perspective of the lens (be it wide angle or telephoto) at 50mm viewing angle most approximately represents what the human eye can see. Practically speaking that is, technically it is a little different.
6. It keeps you "nimble;" both physically and mentally. A prime lens, one that does not zoom, requires you to think a little bit more about the framing of your shot and then makes you move your feet to achieve that framing. Both of which are a good thing, as Martha Stewart would say.
So, should you own a 50mm lens? That is up to you and your wallet, but for me, I own two of them. It is good to have a spare.