Once you've established a good relationship with a chemical manufacturer, you can get a great deal of repeatability in acquiring chemicals. For me, I can't find that sort of situation when dealing through the black market.
I don't like buying street drugs because it makes it difficult to employ the safest practices as possible (dealing with known quantities and known purities- though that's not to say that anybody can assume any RC is pure, but I personally have greater faith in my own suppliers than the random people I meet through the local drug subculture).
So, in that respect, rather than get "acid" which could be any number of things and at any unknown dosage, I can take a titrated, exact dosage of DOM, DOC, DOI, DOB, et cetera. Rather than take an "e cut with 2cb" I can take exact dosage of 2CB, 2CC, 2CI, 2CE, 2CT2, 2CT7, 2CT21, 2CP, etc.
Not only that, but I can store a lifetime's supply of all of them in a safe place. Having that around is also a big factor. I'm somewhat of a drug collector... With multiple samples of at least 50 psychoactive substances. It's like being a living museum...