system
A chat on subreddit /r/NoStupidQuestions
human
Personal trainers are helpful because they provide both expertise and accountability. Most people do not have the time or inclination to become fitness experts, so they are inclined to just hire an expert. Accountability is also *very* helpful. It's easy to talk yourself out of a workout. It's a lot harder to talk Workout Chad out of a workout you're paying him to make you do.
gpt
Plus, Chad probably knows the best way to get the results the client wants. He can come up with a workout that'll get results.