I don't know! Don't intend to go into the field either way so luckily I don't have to figure it out

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 There was another way in (this may be outdated now) where you got points towards joining by working for certain employers.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

The actor's union is a huge barrier to entry. Any movie that includes a union actor is not allowed to have non-union actors if the budget is over some small amount (like $2MM or something).

The only way to get into the union is to be nominated by someone who is already in. The only way to get seen by those people is to work in adjacent jobs to acting, or get lucky and get on a small film with both union and non-union actors.

Once you're in the union it's great. It's a great example of how to form a union for creative people that mandates minimum pay and working conditions but no maximum, allowing for stars to get huge checks while making sure everyone gets good conditions and living wages.

It's supply and demand. Kids don't grow up wanting to be CRUD app developers, they grow up wanting to be game developers. This means that working conditions are notoriously bad (bad pay, terrible hours). To raise pay they need to constrict supply, to constrict supply they need to put up barriers to entry.

Actors union is a definite barrier to entry: you need to either be a notable independent actor/performer (rare), get a job on a SAG AFTRA because you have a talent that they can't find in union (rare), or show that you've worked in the field (non-union commercials/films) enough to be notable. Then you need to apply, pay $3k, pay ~$200 a year dues, and give up ~1.5% of all your contracts to the union. Then you can't take a non-union job (again restricting the talent pool).

I honestly don't see a ton of difference between the two: both establish choke points and collective bargaining in high demand fields to improve working conditions for the people that are in the union.