It's not only, or even mainly, to do with qualifcations. It's to do with accountability.
Lets take a breastfeeding counsellor. If she is part of an organisation which regulates her practice, which ensures she has sufficient in-service training for updating, which stays up to date for her (by informing her of changes in guidance, any important risks her clients should be aware of, etc etc), then her clients are, to some extent, protected. If she gives unsafe or poor support, or upsets someone, or fails to turn up when she says she's going to turn up etc etc, then there is a supervisory body to complain to.
They can then investigate, offer training if needed, check she is up to date with her supervision stuff, and if needs be, actually de-register her and prevent her from practising (at least under the auspices of the organisation).
That, to me, is the important bit - it's a question of integrity and safety.