I started writing snippets of novels when I was a teenager. For years I collected bits and bobs of ideas - many of them which would never get anywhere, a few which got to 15 or 20 pages - until the inevitable hard drive crash wiped a bunch out. I'm still bummed that some of those ideas got lost; sure, the writing was probably pretty bad, but it's the *idea* that I miss.
Since then, I've made sure to keep things regularly backed up in a safe(r) location, and years later I'm once again sitting on a folder full of story ideas. A surprising amount have 20 to 50 pages worth of text, though there's plenty with just a few pages, or even just a few sketchy ideas and no real writing done yet.
This has led me to realize just how hard it is to actually finish something. Not because I get stuck partway through (though that happens) or I lose interest in the initial idea (yep, that happens too) but more often because I get another new idea and zoom off on that one. This means that eventually I go look at my writing folder and realize that I don't recognize half of the things I wrote about until I reread them and remember "oh yeah, that's what I was writing about!"
Of course, the reason to keep these around is that sometimes I *do* go back and work on things again. Occasionally I'll go through my files, see which one pulls at me the most, and throw a few more pages at it to see what sticks. Sometimes I keep going on it, sometimes not.