The Wii is a very social console. From the marketing campaign to the pack-in, everything about it is designed to get a bunch of people together and interacting in the same room. It can even be fun to have a group together for single-player games, with everyone helping out and enjoying the events of the game together.
But video games are not always social events. Sometimes you just want to get immersed in the game without all the interruptions and talking and people. Are you the kind of gamer who prefers to be alone with your games? Or do you always prefer to share your experiences?
Or are there certain games that you just can't stand having people in the room while you're playing? Killer 7 on the Gamecube, for example, was something we preferred to play alone because we didn't think others walking in would have a clue what was going on. Especially since we didn't.