Although there are a few people who sometimes use a longer lens for Street Photography, there are far more that dismiss a longer focal length. The internet is filled with posts telling you to buy a 28mm, or a 35mm or at most a 50mm lens. Sure they are all good options (who doesn’t like a nifty 50) but it doesn’t have to be that way.
The fact I don’t have to stand so close to people is the one over arching thing that makes a longer length so great. When you’re just starting out, or still learning like I am, a 85mm length is great. Allowing you to get the up close and personal shots that make street photography so great, but not have to deal with the people that go with it.
Some of this is confidence, I have absolutely no motivation to go up to someone and ask them if it's OK, nor would I ever point my camera at someone right in front of me and a longer focal length means I am less likely to have to. When some photographers talk about the candidness of a tiny little lens, I get the same feeling with my 85mm f/1.8 Sony lens. Taking pictures becomes easy and worry free.
Typically, photographers think a 50mm lens would be smaller too, but not any more. If I wanted to keep a fast lens in 50mm I would be lugging around the SEL50F12GM at 778g (not to mention the £2,099 price tag), or loose around a stop of light on the SEL50F25G at a fantastic 174 g. Whereas the SEL85F18 I have strikes a great balance at 371g and a very affordable £400.
Using my A7c and a 85mm prime is the perfect combination of nice and light with enough ability to capture the shots I really want and be a little more removed from the subjects. This may change with confidence and progression, but let it be said that you simply do not need to buy some expensive glass nor stand really close to people to get great shots. I love 85mm for almost everything, and you might too.