I get this question a lot. The people who need a trainer are those people who haven't gotten in a habit yet of working out. Also when they do work out, they don't push themselves as hard as they need to in order to see results. In addition, they need the extra accountability of someone weighing them in weekly and to be a nutrition coach.
Even regular exercisers need a trainer. For example, I got a personal trainer even though I was going to regular exercise classes because I needed more information on nutrition, accountability with it, and how to feel comfortable in the weight room. I wanted to know how to do the exercises correctly and not look stupid. I never knew at that time when I was a school teacher, that I would be helping people like I was helped. It changed my life and I know I can help you change yours!