Protection of incumbent union members from masses of competitors.

Acting, stage acting, not Hollywood, is actually where I'm most familiar with this through friends. If you want to be an actor, move to NYC, and try to find a role, you get sent to the back of the line at every audition and probably they don't even get to you. There was another way in (this may be outdated now) where you got points towards joining by working for certain employers. That's why some of my friends played characters for an abusive boss at a now defunct space travel theme restaurant - just for points to join the union