It really depends on the purpose of the mask:
For some, it is a way to "fit in", because they have that need
For some, it is to protect themselves from the world. If you show your true self, you may let others in too deep, and thus lose what you are trying to protect.
For some, it is a game. They can play with this variations of themselves, and see how it effects different people.
Some don't even know they have a mask. For those who say they don't wear a mask, I would challenge that, including myself. It is a defense mechanism. You survey your surroundings, and sometimes you have no choice but to alter yourself, even if slightly, in order to move on. For instance, you go to a job interview, and even though you don't lie, you don't let everything out, thus, a mask of sorts - one that contains the rest of you.
And some are just plain a*holes.