In the early days, we have some casinos that become so popular now already, and they started without a license IMO, but still people trusted them, they risk their money by gambling to their platform, so it's more on the reputation that really matters but since the market has expanded and good competition has been created, license is now necessary in order for site to compete with others.
Licenses are needed only to authenticate how genuine their business is and whether it is actually located at the place which is stated there. People risk their money in all the casinos where they get some freebies or bonuses and they are too lazy to check about any license available on that casino or not. We must boycott websites without license because that is the only way we can stop scams.