Granted, there is a difference between a temp agency or a third-party recruiter and a “coaching” type service. The former are strictly about finding applicants, and they get paid for that service by the prospective employers. The latter are about 1-on-1 coaching, CV editing, etc., and hence they’re paid for by the prospective employee.
That said, I’ve never used one of them nor do I really see the point, given the wealth of information available for free out there. Then again, I did benefit from speaking to career advisors at my university, which were free to me at the point of service but obviously still paid for by me via my tuition.