The service is completely based on no-questions asked refund policy, as long as you have the receipt. Consider you don't like the game, or it doesn't meet your hardware requirements.
Remember when you used to rent a game dvd to test it out on your pc or x box, and if you didn't like what it had to offer or didn't run, you could just simply return it or get another one that does.
Well, up until now you had no such online services, but guess what? Call it a marketing strategy or just a method to come closer to their customers, but Steam Online digital store now provides complete refund service on almost all kinds of items bought from the online store.