the short answer
Online fitness coaching is worth it if your gap is consistency, accountability, or programming - a personal plan, regular check-ins, and a human adjusting your training beat any app for people who keep restarting. It is usually not worth it for self-motivated intermediates, complete beginners who need hands-on form correction, or anyone the monthly fee would strain.
Written from inside the coaching industry, for both sides of the decision - the person considering hiring a coach, and the trainer wondering whether to become one. Figures are reported market ranges, not guarantees.