The big thing about it is that it is untraceable. I read about it before. Can't remember exactly how it is done, but the coins are used to purchase illegal things and keep the buyer anonymous from authorities.
It should be pretty easy to access via the TOR network, which in itself is easy to access.
I've often thought about accessing the 'underground web' but I heard a horror story about a guy that used it once. However the story was probably bullshit and the network probably 100% safe.
But the silk road, 70% of stuff for sale there is narcotics. Pay with bitcoins, get the gear posted to you.