rabbits are generally used for time trials/record attempts. if a runner wanted to try for a 4 minute mile, they would have a rabbit run 2 or 3 laps evenly, at slightly better than 4 minute pace (say 800 in 1:58 or 1200 in 2:57). this allows the other racers to draft off the rabbit, as well as focus on running even splits so they have as much energy as possible for the final sprints.

rabbits in marathons are pretty rare (to my knowledge?) though