Before talking about the 85mm, I want to bring up some different focal lengths. Just to get us warmed up and give you a little backstory to why I love the 85mm so much.
So most photographers probably started off with a 50mm, the nifty-fifty. Super cheap and affordable. Closely mimics what the human eye can actually see. So it's extremely natural. Me myself loved my 50mm. I had the nifty-fifty then moved onto the Sigma Art 50mm f1.4 for my Canon 5D mark ii. Definitely a hefty lens compared to the old one I had but oh lordy, the sharpness of it was awesome. Off topic a little sorry. But the 50 is a great focal length, not too wide, but in my opinion..a little too wide for portraits at times...
So now enter the 85mm. The 85mm produces a nice compressed background. What does that mean? I know it gets mixed up a bit and i was honestly a bit confused as well in the beginning but it made a lot more sense when i figured it out. Well in words, it means that for the same image, you get less background. It's more easier to explain it by picture.
This picture was shot with my 24-70 GM Lens at 50mm. I framed this plush cat right dead center of the picture. Now looking at the picture you can see that in the background, you see the whole guitar and part of the cat tree on the right.
Now take a look at this picture. You can see that the cat is still sized the same (I moved back a little to make the cat the same size). But if you look at the background, the cat tree is completely out fo the frame and you can see only half the guitar. To demonstrate whats happening to the background using a longer focal length, take a look at the outlet next to the leg of the chair. You can see if being moved/stretched outwards. That's what longer focal lengths do and that's what the 85 does.
So why do I love the 85mm lens? It's a fast prime lens that I can keep on my camera through a whole portrait shoot and do completely fine. It gives me the ability to get really tight headshots and portraits without the distractions of the background and really concentrate on the subject. Also most 85mm lenses are going to be at a wide aperture ( f1.8-f1.2 ) so you can really blur out the background and separate the object from the background which helps just as much. If you want my recommendation on a portrait lens. An 85mm will definitely be at the top of my list.