No wonder incumbent members of the occupation or profession are willing to lobby so hard for it [Morris Kleiner, Cato via Arnold Kling]:
Our empirical analysis finds that after controlling for observable heterogeneity, including occupational status, those with a license earn higher pay, are more likely to be employed, and have a higher probability of receiving retirement and pension plan offers. According to our estimates, where governmental licensing is required for the job it raises hourly wages by about 8.4 percent.