I know little of pharmacology but I can see one use of pure compounds: further research. Analyzing how psilocybin affects serotonin receptors could potentially help design other drugs that act on those receptors without psychedelic side effects. It could also help with creating other psilocybin analogs with different effects. And of course having the ability to study in detail the effects that other substances in the psychoactive mushrooms have might help identify other areas of study.
As a software analogy, this is like the debate whether it’s better to have Google Maps vs mapping software and all the underlying data. Of course it’s better to have the latter even if some romanticize the former.