As the name suggests, this type of bonus allows you to place free bets equal to the value of bonus. For instance, if you got bonus worth $100 at the time of sign up, you can place bets worth this amount in various games. This is really good option for trying new games on various casinos. You might get lucky and make some money without the need to invest anything.
Did any of you made huge money with this bonus amount ?
I once came from the faucet money on Primedice up to 30k satoshi. I guess it was a big success in comparison with what the faucet offered, but, you know, it is still not that much of money. I also registered once on a new (at that time) casino website that offered a big welcome bonus to new users. It was something like $100 worth of their specific tokens. The problem was that nobody confirmed withdrawing any money from that website, even though there were some posts in that thread asking for such confirmations. Playing there became useless fast, as that website probably didn't allow to withdraw money.