Can you be more specific with your question? I have never heard of fitness instructors performing fitness tests since it would be outside of their scope of practice…
If you are referring to fitness assessments, usually certified personal trainers perform those to have a starting point for clients with regard to creating an exercise program. It helps them understand any joint issues or weaknesses in the body before developing a program for them.
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!