The purpose of it is to make sure you can function fully as a member of society in the gender role - socially, career wise. They are trying to ensure your safety and well-being, not end up ruining your life making snap decisions about irreversible changes. I tend to agree with Melissa in her last post - RLE by definition means living, working even sleeping in the gender role you're transitioning to - going out and actually pursuing that which you seek, not shying away from the challenge or staying in the closet at home playing a fantasy out in your mind. I started mine myself before even approaching my GP - my name was changed long before my first appointment at a gender clinic.
There is a lot more to living as a man or a woman than just how your external features look - you might be sure of what you need, but the people that can help are not going to take such drastic measures unless they are certain, that you are certain and capable of living.