Like said above, depends on the part..
If it's desirable and I don't need it, I'll sell it. Otherwise it'll go to the trash or scrap yard. I have spare (used) inner and outer tie rods, lower ball joints, completed left & right front spindles, flanged slip yoke, completed left & right rear hubs, and various other little things. I only carry the small parts with me when I'm on a wheeling trip. Having a spare tie rod, even if it's an old one, comes in more handy than you would imagine to limp off the trail. Other larger components are good to have spares b/c say If I needed to replace a front/rear wheel bearing I can have the one on my spare repaired which minimizes the downtime of my vehicle.