A couple months ago, while writing one of my posts on Mario & Sonic at the Olympic Games, I realized that forcing the player to unlock certain events in that game was a very interesting way of making the player play the game long enough to get used to the controls and learn to enjoy playing the game. As I stated in that post, the game takes a little getting used to, and using the dangling carrot of unlockable content to coerce the player to practice at various events enough to win gold medals was a pretty good method of solving that problem.
In one sense, this use for unlockable content is a sub-set of the more general purpose: rewarding the player for putting a lot of time and effort into the game. A good example of this is Disgaea, which has at least two characters that can be recruited by diving very deep into the game's high-level post-game. While beating a game and watching the credits roll can give a player a lot of satisfaction, most games nowadays come with a lot of optional content, and many different scales of difficulty. Accomplishing a very difficult task and getting no kind of reward for it is a very frustrating experience for a player. Unlockable content generally works very well as a reward like this.
A very good example of a game that uses unlockable content right is Devil May Cry 3. Devil May Cry 3 is a pretty straight-forward action game. Once the player has beaten the game, there is not much more to do in the game except attempt to beat the game again on a higher difficulty level. As a result, the game has unlockable costumes, game modes (new difficulty settings and Heaven or Hell mode), artwork, and even a god-mode (Super Dante). In Devil May Cry 3, these unlockable features serve the main purpose of rewarding players who put in the time and effort mastering the game to the degree required to beat such a hard game on its hardest difficulty settings. They can also serve as bait, to try and get players to attempt higher difficulty settings.
So then, what is the point of making a significant portion of the cast and stages in Brawl unlockable? I think in part it is marketing. There has been a lot of buzz built up over the last year about which characters would be in Brawl, even to the point where people created hoaxes to fill in the information gap. Having secret characters was a good way to help build this speculation and interest in the game. The rest of the reason is reward. Most of the stages and music are handed out as rewards for players who take the time to clear event matches and other optional content in the game.