There's a couple of different kinds of lenses out there. The two specific distinctions are zoom lenses and prime lenses. The difference between them is really simple. Prime lens is just one focal length. In this lens's case this is an 85mm portrait lens. This is a zoom lens and its focal length is 70-200. Within this lens is the same focal length as this lens. So why would you ever use this lens when you can use this lens? And the answer is pretty simple. It's just a matter of preference. This is obviously a much bigger lens. There's more to carry around. This is a lot more small and compact.
Now a lot of professional photographers actually choose to use prime lenses for a couple of different reasons. One is just one focal length that you kind of get accustomed to. You become really comfortable with. But another reason is that prime lenses tend to be a lot sharper than zooming lenses. And that reason is because there are no moving parts other than the focus. So you can get more glass and it gives you better overall image quality.
However, if I was shooting something like say a basketball game, this really wouldn't be a very good option. I would want to bring in this guy instead. Players running up and down the court, you want to be able to zoom in and zoom out with them, and get them in the center of the frame.
A lot of old school photographers coming from the film age also swear by prime lenses. A lot of these really famous Magnum photographers, guys who are known Rangefinders, like Leica's all swear by primes. Just because again it makes it a little bit more of an art. You're moving your body around instead of moving your lens around so you're actually physically seeing things differently. Rather than standing there and zooming in and zooming out.
Personally I'm a prime lens shooter. I like the focal length 85, which is this, and 24mm. I do also on occasions bust out a zoom lens. Another important thing to keep in mind when looking to purchase either a prime or a zoom is prime lenses will always be a fixed maximum aperture. The aperture being the opening in the lens that lets light into the camera. A larger maximum aperture, which is actually a lower number, this one is a 1.8, means you're going to be able to shoot better in low light. Primes generally tend to have a higher, in reality lower, maximum aperture. Something like 1.4, 1.8, 2.8, those are really good maximum apertures.
This is a 2.8, where this is a 1.8. So this will do better in low light. But this is also a very good lens because it's a fixed 2.8 from 70 all the way to 200. However, as you get cheaper lenses or more affordable lenses, a lot of times that low maximum aperture will actually jump up as you zoom in. So it's not uncommon to see something like a 70-200 like this, that instead of being a 70-200 2.8, it will be a 70-200 3.5-5.6. Which physically means that that maximum aperture is getting smaller and smaller as you zoom in. Not super great for low light.
For that reason, zoom lenses can be a lot more expensive and do a lot worse in low light situations. You also have to keep in mind that there are a lot more moving parts on a lens like this than on a lens like this because there is nothing zooming here. In addition most prime lenses don't have things like image stabilization that you'll find with lenses like this. This is specifically made for sports photography. So it has optical image stabilization. This does not.
Again it depends on your subject and it depends on your matter of preference and it depends really on your pocket book.