Why Do Racehorse Trainers Restrict Performance?

Question
hello sir, i have observed trainers make few of their stable horses perform lower than their(horse's) ability in the race. may i know why?

Answer
hi well sometimes a horse is not ready to perform at their best because it is coming back from a rest or a operation etc. they want to make sure the horse is up to a great performance before pushing them in a race. other times the trainer is trying out certain equipment in order to see how the horse  responds to that type of changes. i do not think that they are holding the horse back in a race for no reason. thanks ED HESS