Sign-up bonuses are extremely popular because there is nothing like a juicy bonus to use when you visit a casino for the first time. The sign up bonus is used as a marketing tool by online casinos to promote themselves and bring in new customers, thereby increasing their player base and building their brand. It is usually offered in the form of free money, bonus money, free spins, and more. A sign up bonus is an incentive that online casinos offer new players to get them to sign up and play for real money.