Downloadable games, especially the small ones that cost five or ten dollars, are fun to buy, and often give a good throwback experience. It's also extremely convenient to have a bunch of games stored on your hard drive. The very fact that these games are inexpensive make them viable for the impulse purchase. And considering many people will make their purchases based on nostalgia, it can be tough to fight the urge to download a game.
But don't be so hasty. Sure, impulse can be a good thing. You may buy something you really like that you would have otherwise overlooked. That's how I discovered Pixel Junk Monsters, after all. But when it comes to games that we've played before (some of them, decades ago), I think we need to evaluate our purchases more carefully.