I don't understand the point of using TOR to access coinbase if you are buying bitcoin with coinbase. You do realize that you needed to verify your identity prior to them processing your first purchase or sale of bitcoin, right?
The only reason why I can imagine someone wanting to use TOR to access coinbase is if you are either using a stolen identity or a hacked account to purchase the bitcoin.
Not neccesarily. Some people just want to remain anonymous, though obviously some criminals also want this. I don't think you can sign up to btc-e through tor either.