This ploy suggests that the length of time a remedy has been used is a measure of its effectiveness. Its promoters imply that if the remedy didn't work, it wouldn't remain available. The falsity of this ploy is easily seen by noting that astrology has survived for thousands of years with no reliable evidence of any validity. Their writings may list dozens or even hundreds of publications that supposedly support what they say. This is probably the most powerful slogan in the quack's bag of tricks.
People generally like to feel that they are in control of their life.
Quacks take advantage of this fact by giving their clients things to do - such as taking vitamin pills, preparing special foods, meditating, and the like. The activity may provide a psychological lift, but believing in false things tends to carry a high price tag. But personal experience is not the best way to determine whether a method works. When someone feels better after having used a product or procedure, it is natural to give credit to whatever was done. For these reasons, scientific experimentation is almost always necessary to establish whether health methods are really effective.