To make the point clear, there are a few items I have in my collection that I stare at in confusion and ask myself the question of the subject title. Not quite so much having something misplaced as not remembering I ever had it or never having wanted. It got me curious as to whether or not I'm alone in this regard.
A few personal examples are as follows:

GDO Swerve. I can only guess that it was the character's appearance in MTMTE comics that even drew my eye to this purchase at all, but even so I can't imagine what I ever saw in this repaint of the Kup mold that would really satisfy that itch. Especially now with the upcoming release of Legends Swerve.

HFTD Terradive. As a general rule I tend to stray away from Movie figures unless they adequately fill a certain void in my Classics collection, so to have this guy is just... mind-boggling. I mean it's not all too bad in it's own right and I like the alt mode, but I can't imagine what in the world was on my mind when I got it.

Animated Grimlock. Another rule of mine is unless a new series feels like something I can really get in to I'll typically just get the usual four and be done with it, and even as much as I liked Animated the aesthetic in toy form just didn't grasp me. So I stuck to the usual plan... and ended up with more than that anyway, because there were just some figs that were just a bit too good to resist. Despite all that I still wonder... what the crap drove me to getting Grimlock?!
At the end of the day I blame my brother since we pretty much share a collection and any oddballs I find stacked on the shelf I can usually blame on him, even if they weren't actually his fault

But really, has anyone else had a conundrum like these?