Take, for example, my recent acquisition of a WWII era Speed Graphic. Photo here:
Now, I already have the Graphic View camera, so why did I need this? Well, of course, I didn't. But I wanted to do a project and this camera was necessary.
Historically, this is a cool camera. It weighs a ton and I don't really know how the journalist photographers back in the day could lug this thing around and actually get photos of any action. It's not conducive. But that's what they did and most every photo in a newspaper or magazine from the early 1900s until just after WWII would have been taken with a camera like this. I like this history.
But, my real purpose was to use an old lens that I've been carrying around since 1980. In my grad school days in an optics lab, I rescued an old war surplus Kodak Aero Ektar aerial photography lens that we had bought for cannibalization purposes. The optics were great, and I could not bring myself to see the thing go in the dumpster, so I saved it. I always wanted to use it to make a large format camera, but never knew how to start.
Recently I realized there are other strange photographers like me who also have AE lenses, and they found a way to retrofit them into the Speed Graphic cameras. See, the SG camera has a focal plane shutter in addition to the normal leaf shutter in the front lens, so an AE retrofit without a leaf shutter will still work.
Great, you say. So what? Why go to all the trouble?
And the answer is: Portraits.
The AE lens on the SG body gives you a 7-inch focal length with f/2.5 speed onto a 4 x 5 inch negative. That means very small depth of field, perfect for portraits. Plus, the positional adjustments available with the rails on the SG camera allow some funky control over perspective and focus area. Look at this photo, for example courtesy of (https://www.facebook.com/macieklesniakfotografia?fref=photo):
By playing around with the positioning of the lens board with respect to the film plane, you can control what areas of the image are in focus and which are not. Here's another more extreme example courtesy of (https://www.facebook.com/kornel.kabaja.portrety/photos/a.617290181622240.1073741828.617195194965072/1093089077375679/?type=1&theater)
OK, so maybe you don't care about this stuff, but I think it's cool and I wanted to play around with it some. So, I take my 50 year-old lens and my 70 year old camera body and make some interesting photos. That's the definition of a hobby, to me.