Sooner or later, if you shoot enough, you'll find perfectly good photos that don't fit into any other category. Like most other photographers, I tend to just stick them into a slide tray just so they don't get lost among all the other photos that don't quite make the cut.
Normally, when I scan photos from a slide tray, I don't bother doing much more culling. After all, if I bothered to stick them into the slide tray, they met some prior standard. But because this tray saw so much evolution over the years, I made an extra pass just to throw out stuff that I never would show others at this point, when my standards are much higher than when I first started out.