No, you don't.

In some circles of photography, the 50mm has reached almost cult religion status. It seems to solve everything from focus problems to technique stagnation to global warming. Ask a question about photography, and chances are, someone will recommend a 50mm lens as the solution ("I want to take images of the space station crossing the sun" "The 50mm 1.2 is perfect for this! the narrow depth of field will give you a nice soft focus on the sun's surface")

In reality, for some types of photography, the 50mm is awesome; some portraiture, but more still life and item photography and places where the narrow depth of field can get the image a very strong focal composition. And if you do that kind of photography, then a 50mm is a good lens to add to your kit.

but many of us see no need for it, don't do photography that lends itself to a 50mm, and have no intention of ever buying one. And while we get shunned by some photographers who look in our kit and see the missing slot where a 50mm should go, we don't feel particularly deprived.

There's a lot of "religion" tied to the 50mm left over from the days when prime lenses were significantly better than zooms, when film was slow and cameras were manual and lenses were slower and your choices were limited. Today, it's a lot different landscape than it used to be, and so some of these "givens" aren't really as true as they used to be.

So here's the hint: anyone who tells you to get a 50mm without knowing what kind of photography you do or how you plan on using it is wrong. it's not a panacea, it's not a given it'll improve your photography. It might, depending on your interests.

But it's certainly not a given that you need one.

So here's the question back: what would you use it for? How often are you shooting your existing lenses in the 20-80 range wide open and what are you trying to do then that you find you can't? If you aren't fighting to create an image under those circumstances and failing because your lens won't do what you need to do, then a 50mm probably won't solve the problem you're trying to solve. It might, however, be an interesting lens to add if you want to start exploring new techniques or styles of photography -- but either way, don't buy it because someone said to, buy it because you've analyzed what you're doing, thought about how you'll use it, and decided that it will help you do whatever you are wanting to do with your camera (that your existing gear can't).

I always tell people to never buy gear unless they can explain what that gear will do for them that their existing ear can't (or can't do well, or easily enough). too often we get into gear lust, and too often, that's what ends up gathering dust...

(by the way, if you have an aps sized (or crop) sensor, it's not even a 50mm any more. that works for full frame sensors, but if you have a crop sensor camera, it's about a 35mm lens... 50mm on a crop sensor acts more like an 85mm lens).