Canon-wise it doesn't really matter if you can select between male or female if the main story of the game plays out the exact same for both. At the same time, it brings more replay value, and an option for people to choose a gender that they might more easily identify with. After all, there is quite a bit of female gamers in the world these days, and while I'm sure not all of them care if they control a male or a female, I am sure that a lot of them would like to at least have the option in a world where they most of the time have no choice but to play a guy.