Like most long-timers, I keep all parts from kits that arent used. Way back when, before the internet and when FSM still had matte paper pages, Paul Boyer did an article on the subject and I took it to heart. After all, if Paul said it, then it was good enough for me!
I also keep all parts from damaged or "boneyarded" kits, too. Over time, I remove kits from my collection or just simply decide that I screwed one up bad enough that there is little sense in keeping it any longer. These are dissected and cut apart, with all the parts being retained and cataloged
I have landing gear of every kind, propellers, cockpits and enough mini-ordnance parts to start a war. There are also heaps of wings, tail surfaces, panels and sections of fuselages, flightline gear and an entire army of little pilots, ground crewmen and whomever else may have populated this kit or that one. I keep this "inventory" in one of those slide-out-drawer type cabinets. In one of them, I think I still have one of the original Preiser 1/72 Luftwaffe Personnel sets, each of the little Schwarzemaenner und Piloten prepping for their next sortie.
Sometimes I just prowl through the drawers, admiring all the little fiddly bits, marveling at my apparent skill. Each little trinket tells a story, and its neat to test my memory and try to recall which kit a given part came from. Sometimes I go rooting for a pair of wheels or some cockpit doo-dad to add to a current build, like I just did with an LTD Yak-9, which had awful wheels. In fact the Yak may end up in my "spares inventory" itself, as the wing root fit is so bad that I have rejected it from bench, as of this time.
I also scrounge the drawers for an odd or end for other models, seemingly out-of-place bits that actually work in context. Some 1/35 armor pieces can find new life on 1/48 airfields and vice versa, or an old tire or rim can fit into another vehicle build. For instance I have the old AMT 1952 Ford F-100 in my stash, which I envision as a "beater," a working truck complete with a welded pipe/parts rack - and a bed full of tires, rims, boxes, tarps and other assorted junkyard flotsam. Guess where it will all come from? The spare parts drawers of course!
So yeah, I keep everything from models. I suggest you do the same. After all, modelers tend to be packrats in the first place. So next time you are considering another addition to your spares box, indulge yourself, and proudly exclaim the Pack Rat's Motto:
"You Never Know..."