I answer this question from the perspective of client retention of a personal trainer, not gym membership.
In addition to many great answers already given, I believe it is the ability to modify a client’s workout when unexpected problems arise. Exercisers often fall off the bandwagon when they get hurt and feel they need to stop exercising altogether. Why this is sometimes necessary, it is not the default. Being able to come up with workouts that work around problems and maintain the momentum of exercising has been a client retention toll for me.