Apart from keeping hypochondriacs and other attention seekers out of gps' offices, what good does quackery do?

Warmth. Some of these practitioners are self-deceived, and all offer warmth, caring, concern, attention, and time that in our dysfunctional health care system, physicians cannot offer so extensively as the charlatans do. It's what keeps them in business.
Placebo effect. If the treatment only takes money and does no harm the worried well person may benefit.