Ok. You are partially correct but it does much more.
Sure weight transfers forward when you get on the brakes but trust me when I tell you that when you trail brake you are doing a couple things.
1. Putting forks into their working range.
2. Enlarges the contact patch of the front tire on the pavement because the weight is transferred forward. There is friction between tire and the road whenever the tire is touching the ground, even when straight up and down. The size of the contact patch determines the maximum level of grip that can be used, whether it is for brake pressure or lean angle. Think of it like this, you only have 100 points of grip in any tire. You must manage that level and if you go over 100 points of grip you slide and or crash.
3. Provides direction because speed = radius.