No-Deposit Bonuses exist as an enticement to get would-be players to sign-up for online casinos, and at their face, they offer free value to the player. Players do not have to make a deposit in order to play such a bonus, (but may have to make a deposit to be allowed to cash out) so these bonuses give a player an opportunity to sign up for a casino and play with the potential for winning money with absolutely no risk to them.
When it comes to expected value, many online casinos offer Deposit Match Bonuses (and other types of bonuses) with a much greater expected profit than that of No-Deposits. For that reason, I would tend to often avoid No Deposit Bonuses