Similar to what the previous fitness professionals mentioned, trainers use fitness assessments to get a better understanding of the current state of their client’s health and fitness.
This gives a trainer the ability to create a program based on results rather guess and check. More importantly, regularly re-assessing your clients health and fitness gives you the ability to provide meaning by progress data.
If you are looking for an easy to use tool to manage your clients and track assessment progress you should check out http://www.trainermetrics.com
I hope this helped!