Most people do it because they have no self-control but you'll rarely meet an honest one that will actually admit it.
This is the only answer.
And to the guy that said he does it because he's not poor, what an idiotic answer. No matter how rich or poor you are, there should be a purpose to what you spend your money on. I want a tablet, but realistically I'll only use it a few times a week for 10mins total. Therefore even though I can afford to buy multiple tablets out of pocket, I refrain from doing so as it serves no purpose other than to say, "Hey I own a tablet."
Sales. How could I pass up deals like Forza Horizon for 15 bucks, Binary Domain for 10, Lollipop Chainsaw for 10, and so on. It's always good to have several options depending on what type of mindset you are in.
"Hey look, i got too much money and i waste them on games."
Do you often decree what is a waste of money, when it's not yours?
Right now, there isn't a game in my collection, played or unplayed, that I would trade. There is not one regretted purchase. As I said, I bought these games because I want to play them. My money was not wasted.
On sale is the main reason I do it, especially when I know it's not going to be any cheaper or if I'd play it right away, like I have a backlog of like 50 games but I bought batman GOTY edition for 30 bucks on boxing day since it was on sale, it's an insanely great game and been playing it nearly non stop lol.