Certain bookmaker will provide you with a free bet up to certain amount for you to bet. One way that bookmaker provide no deposit bonus are similar to a sign-up or reload bonus. Bookmakers will provide you a free bet equaling a certain percentage of your deposit. They might provide a free bet at a preset amount after you meet a certain deposit threshold.
When a free bet is provided in this manner it can usually be used on any game or match of your choice.
The game and match on which the bet can be placed isn't always the bettor's choice; sometimes it must be placed on a certain game. In this case the sportsbooks will provide a bet at a predetermined amount if you open a new account and/or deposit a certain amount into your account. There are usually limited time promotions and players must make their deposits prior to the start of the game of match.
Free bet bonuses aren't only for new players. The sportsbook typically offers a free bet bonus to both new players and existing players. Free bet bonuses are often provided at the beginning of the season or tournament in an attempt to peak a player's interest.
Just like every bonus that sportsbooks offer, it is a win-win proposition for the player. They receive free money to use on a chance to win more money. It doesn't get any better than that.