its a middleman currency created for the sole purpose of trading for illegal things on the hidden web.
so to keep yourself anonymous from whoever you are buying from.
the exchanges and other ways to spend them on the normal web is just all an inside joke so to make people think they are trying to run a legit net currency.
the mining stuff is just your comp being grinded from across the net, to perform processes elsewhere, and you are paid a tiny bit of bitcoin for it.
its not much different then filling out surveys on the net for 2 pennies each, or clicking adds for those referral programs.