Lusche, a former trainer, gave a nice explanation.
Another factor is the need for specific information. Currently we lack expertise in ground vehicles so that's something we're looking for.
Helping people in the game, the TA, and the BB, gives us an idea of your knowledge, willingness to help, ability to communicate, and character.
I never tried to be a trainer, I just helped people because I like to and because it helps me to learn.