I keep everything because of the few (and I do mean FEW) instances where I've needed something and I knew I had it in a box somewhere.
Case in point: 10+ years ago I bought an apple peeler/corer. Used it once or twice and put it in a box. Now that I make apple pies several times a year during apple season, it would come in VERY handy.....if only I could figure out what box it's in.....