Not being in that field, I can only speculate. I would imagine its the joy of helping people feel better about themselves because you make them look better. People often say looks don't matter, but that doesn't mean people don't feel bad when they don't look like models, so being one step closer is a confidence/ego/self esteem booster.
I would think that is the best non-monetary reward for the hairdressers and beauticians out there. I'm more than happy to be wrong though, as I said, I am not in that field.