Steam sales are an important piece of this. I always buy a bunch of games from Steam sales, knowing full well that I'm not going to play all of them. The things is, I don't know which games I might feel like playing in the future, so I get several. I don't buy them when I actually want to play them, because then they won't be on sale.
99% of the time those games remain unplayed forever though. I just wishlist games I immediately want to play, and buy them and play them once they go on sale, immediately.