(1) Because not all masters are benevolent
(2) Because it removes the possibility of charge backs
(3) Because not everyone enjoys being monitored by companies and/or governments (foreign or otherwise)
(4) Underground markets are only "underground" because of laws. If a society outlaws birth control and abortions, but a women wants an abortion she probably wants an alternative (I.e. something an underground market with provide). An anonymous crypocurrancy (or cash) is the easiest way to do this. If it's not a crypocurrancy or cash, they can always trade goods. So in all honesty, there's no way to stop it.
There may even be other reasons, but it seems pretty obvious it can inherently good to have.