Agreed, but many drugs have a direct effect, not subtle and it's pretty much immediate to know, let say a user try that new sub-q injectable from a shady forum that discuss X compound that is supposed to cure acne, then follow protocols from other users, then do their report, often with photos, it's useful even if it's not standard, as you have a clear picture of before/after usage of that drug (it's probably fair to say that the acne clearing is that isn't placebo effect), it's useful even after a drug is authorized on the market as well as it gives additional user reports of efficiency.

Saying that wouldn't be useful is kinda like saying HN/Reddit isn't useful because it's all anecdotal, that applies to almost all topics in life.

Lastly, let's not pretend that most doctors do actually follow properly a patient blood before/during/after a course of a drug, in most cases, doctor will just ask How did it go, any side effects?... That's a new "data" for the doc, all anecdotal, almost the same value as a "trusted" rando on Reddit.