Before anyone begins to increase their activity level, they should know where they are with regard to their fitness. For my clients, this process is very individualized. Many assessments are too intense for many people. Some are inappropriate at the beginning, but may have a place later on. But, in general, I record certain aspects of every client’s current fitness level for several reasons. Not the least of which are safety, program guidance, and future comparison. I also find that keeping good records of each client’s journey helps me and the client make the most of the process. It would take a long time to explain every aspect. Are you a fitness instructor? Don’t you keep records and assess clients?
The above answers are great for personal fitness instructors.
As both a personal trainer/instructor and a group fitness instructor baselines are important in both areas for pretty much the same reason. It helps people know how they have (or havent) progresses. I do recommend instructors repeat a program every once in a while as some participants might not have been able to complete the routine originally and it is a huge boost to see and feel the progress from the last time the routine was completed.