One that people always miss: Online leaderboards. For some, being there first is the only hope they ever have of placing in one of the top spots, however short said placement may be.
Also, though we might pretend otherwise, a lot of games* are treated less as consumer products and more as mass short-term cultural phenomena**. You buy and play a game to socialize and take part in the experience of everyone buying and playing said game. Sure, you can always get the game later for a lot cheaper, but ultimately you will have missed your chance to participate in the event. You do not get the same sense and feeling of belonging to a cohesive group once a couple months pass and sales dwindle, players stop joining multiplayer matches, and discussion on message boards and in real life shifts to something else. (You can see this by how the overwhelmingly majority of sales for most games come in the first month or two).
For instance, I might not care for the Mass Effect series, but I still rented 3 in the first week so I could play through it and know about/participate in the dialogue over how epoch-definingly crap the original ending is.
Encouraging this sort of thing is especially the model for high profile games (because that is ultimately the only reliable way to generate that kind of sales), and the big developers are doing everything in their power to discourage and destroy that model for reasons of stupid.
*That is not to say that this principle is only applicable to videogames here; most popular media works like this. Major television shows, blockbuster movies, various book series, et cetera.
**see: Call of Duty, Ass Creed, most first-party games. And on the other side of the Pacific: Dragon Quest, Final Fantasy, and Monster Hunter.