Some people have no idea how to diet or exercise, or have no idea that they're overweight, or have specialised conditions that make it hard to follow generic advice. These people might find it really valuable to receive individualised advice and education from their doctor.

Also this would often be in the context of the patient coming to the doctor with a complaint. If the doctor says "trying eating healthily and exercising, then come back in a month for a follow-up", some might just do nothing but many people will actually try it.