Here's a brief list:

1. Non-competes. If you're an actor, the studio that you did your first film with doesn't own you for the rest of your life.

2. Getting paid for your time if you're called out for a shoot, and the shoot doesn't happen, because the director's dog ate the script, or some other non-sense that you have no control over.

3. Getting paid for your time if you're called out for a shoot, and told to hang around for 8 hours, so that you can do your 5-minute scene.

4. Getting paid if you're a theatrical understudy, have learned your part, and are sitting on-call, ready to step in if the main actor is sick/vacationing/etc.

5. Getting paid more for understudying more roles. Someone who understudies two principal roles has to be paid more than someone who understudies one other chorus role.

Prior to unionization, violations of these were incredibly prevalent across the industry. Studios who took advantage of actors were rewarded by the market, and anyone who tried to push back on these practices was black-balled from the industry.

Despite having to pay actors for their work, and not treating actors like chattel, the entertainment industry has not yet fallen apart.

Two examples of actor union rules that come to mind are employers paying for journey to location of its more than a couple of miles, and the (very strict) rules about filming nude scenes (only essential people on set, people standing by with bathrobes etc)

> That’s a good example. I don’t know much about it, what benefits does the union in this case provide that isn’t achievable without one?

This seems like an important benefit:

https://en.wikipedia.org/wiki/Residual_(entertainment_indust...:

> Residuals are royalties that are paid to the actors, film or television directors, and others involved in making TV shows and movies in cases of reruns, syndication, DVD release, or online streaming release. Residuals are calculated and administered by industry trade unions like SAG-AFTRA, the Directors Guild of America, and the Writers Guild of America.

Per the wiki page, these rights were won and have been maintained by striking and union-lead contract negotiations.

Before the actors union stars were made by a studio and forced to work for that studio for long hours and little pay. If you spoke up you were out. Lucy Ball was smart enough to start her own studio only to see it get sold a few years later.

If they didn't have a union movies would look different. Maybe better for the customer because you get to see your favorite star in 10x more movies.

Actors don't die on set any more, so that's nice. From the SAG-AFTRA site:

"SAG-AFTRA members are entitled to a variety of benefits, including contracts/collective bargaining, eligibility for the SAG-AFTRA Health Plan, SAG-Producers Pension Plan, the AFTRA Retirement Fund, the iActor online casting database, and much more."

https://www.sagaftra.org/membership-benefits/member-benefits

I actually think Healthcare. https://www.sagaftra.org/sag-and-aftra-health-plans-merge