"Known deficiency" is a thorny issue in itself. The various serum levels and necessary intakes vary widely by individual. And recommended daily allowances for many kinds of vitamins and minerals are set conservatively low to avoid overdoses from people chugging the bottle.

"levels are fine" can still mean fine, too low or too high for your individual need. As long as you don't overdose, trying out a supplement nonetheless can be worth a shot, but of course it can also be a waste of money.

Unfortunately, "normal values" in medicine aren't as normal as one would like.