On paper, the point is to not get fucked over as you said. In practice, it's a way for them to basically get job security and better benefits, but only for the "senior" workers. The newbies can cry about it if anything happens to them. Small-scale unions are cliques that you fit into or get ejected from, except you're being ejected from the company because you failed their personal vibe check. For large-scale unions like SAG-AFTRA, they're basically an industry of their own that exists to keep out anyone from the industry that isn't in their clique, and if you want a union member in your project, you need to either use only VAs from their union, or advertise all the non-union VAs to them that you WANT to use, that THEY have to approve. And I think you have to pay them for each non-union VA you use, but it's been a while since I read SAG's clause about that shit so I may be misremembering. How could Fran Drescher do this?
In short; Small unions are cliques so studios would want to unionize to form official cliques, and large unions are mafias without the Italians, which want to legally monopolize their "turf" or if they can't do that, get paid their dues where applicable.