I know bands that are very successful booking themselves, and bands who hold themselves back trying to keep it all in house. If you get a recurring gig or become a house band you may not need a booking agent. But a good booker wants to make money too, and will not only book you more but will try to grow you so that you get more for each show. As your fan base grows and you can draw bigger and bigger crowds they can ask for more money. Think about it from the club owner’s perspective, they need a crowd. If you can deliver they can pay more. Also as an artist, you need to focus on your craft. More often than not you need someone who can take care of the business part and let you do what you do best. So for most musicians I say, find a good booker, tell them your goals, and let them help you reach them.