They send them automatically but if you have more than two letters, and a school accepts only two letters, you may want to "target" certain letters to certain schools. If not, LSAC will zip em off in the order in which they received them from your recommender.
This can be important. For example, I have a professor who's an alumni from a certain school, so I wanted to make sure that particular school got his letter.
You can figure out how to target through their demo video on LOR's.