It's a long time since I did it but the TEFL course itself was more about setting you up with basic techniques to teach people who spoke a different language to you plus some rudimentary classroom management skills that you're probably well beyond anyway if you're a qualified teacher. After that it was mostly a case of learning on the job, and that went as far as you wanted it to. The communicative techniques were useful though.
Agree that business teaching is the way to earn money but I think it would be quite hard to just go into it cold with no contacts or market knowledge, which is why a TEFL certificate and a job in a language school is a useful stepping stone.
Most schools have business oriented students - the main reason for learning English is career advancement, after all. And most decent schools would prefer you have the TEFL just so they can say to their paying clients that you do, even if you're already a qualified teacher. It just helps to sell themselves.
Once you're in a school, you can work on building a portfolio of private students from this pool of adults who are in the market for tuition, and then look to see how best you can position yourself. Potentially it's a good money maker.
Alternatively of course you can teach in an international school with your teaching qualification, and the money is fairly good by local standards, most places, and you can have a nice life.
But just working in a TEFL school, with no private students, is good fun and a nice life experience, but won't net you a decent salary really and a lot of employers are not great as their target employee pool is young drifters who aren't looking for much.