Some years ago I worked with a band full time but we employed the services of an agent.The main reason is that the person responsible for booking acts at a particular club doesn't want to talk to complete strangers hawking their band. By using an agent to source entertainment they've got some guarantee of the quality of act they'll receive. You could try sending a demo but most of them hit the bin unfortunately because the recipient is all too aware that bands that sound great on a demo don't always live up to expectation.
We got lots of work through our agent, and were happy enough with his 15%, but for him to make money from representing you he wants you out working at least 4 nights a week which gets a bit wearing if there's any distance involved. Not to mention the fact that HE doesn't care how many flights of stairs there is to negotiate at the gig or what the quality of venue/audience is.
Overall it was enjoyable but these days the band still meets regularly, gigs occasionaly and avoids stairs wherever possible!