I think it is for convenience. Ultimately you do not need to go drive/walk/bus to a store, then walk around in the store/ask the employee where the game is, and then wait in line to make the purchase, and then wait to either swipe your credit card or get your change back.
You save a lot of time purchasing it directly online, and you do not have a physical product taking up space in your house. I personally don't like when it have 50 games, and it takes up a huge shelf, but to each their own. Its art to some people.