It's simply cosmetic. They want to be as they feel, I don't think they believe they can simply be the opposite sex by having the operation, I think they just feel it would make them more comfortable in their own bodies.
But I agree that it might be healthier both physiologically and physically for a person to undergo some sort of therapeutic treatment to accept their bodies for what they are, instead of getting a "sex change" operation. But surgeons are willing to do it to make a some money. In my personal opinion it's sort of enabling unhealthy feelings that come with a mental disorder. Not that there is anything wrong with a person wanting to be the opposite sex, I just feel restoring to a surgery is a bit extreme.