Starting out like most photographers I was weary of prime lenses. They felt restricting and outside the narrow apertures I didn't really understand their appeal. I knew the pros used them but the pros were wrong I had decided. I had picked up a 35mm 1.8 early on while building my kit but I rarely used it, if ever. I was happy with my tele-zooms, and while traveling quite honestly they were better for my needs. But as I started doing more and more portraits shoots my needs changed. I knew I needed a dedicated portrait lens to get a more consistent look. Now there is a lot of talk online regarding what is the optimal focal length for portraits, I won't dive into that here. But for me I chose the Nikon 85mm 1.4D and I couldn't be happier. It is the lens that really changed my mind on prime lenses.
Let's be honest, aperture is a big deal, so much so now Apple is applying filters to re-create that narrow aperture look with their new IPhone 7. With the 85mm 1.4 I'm able to really control where the eye focus's, with portraits it's locking in on those eyes as you would naturally when meeting someone.
It's also about that getting that creamy bokeh in the out of focus areas. Photographers geek out about this stuff. I confess I looked at image after image when researching which lens I wanted (I was close to settling on the Sigma 85mm, another great lens but ended up going with the Nikon and I am happy I did). The Nikon 85mm really just nails it here as well as letting the right amount of saturation through.
Eventually I hope to transition to a Medium format set up ala Zack Arias. I am huge fan of the medium format "look". A big part of which, is the narrow apertures a sensor of that size can produce. But I have to say with the 1.4 I think I will be able to stave off making the switch (at least for the time being, lol).
Depending on the situation each lens has it's place, but for my bag, the 85mm currently reigns king.