In California, at least, it is required. The owner is responsible for the employees' work. The employees don't have to be licensed, so the employer must be. An apprenticeship is mandatory, in California. You have to be employed by a licensed contractor for 4 years and be able to prove it (timecards, etc).

Probably to determine quality of employee and to fully understand the process. It seems pretty risky to just assume you're not gonna screw all that part up