Technically it's a guild, not a union. It's good for things like providing health coverage and making sure writers get paid and get credit. From what I understand, it has not been great at ensuring sane working conditions.
I met up with a pro TV writer I know and he said he was so happy the season was over because he could finally go to a mattress store and buy a new mattress. He said he didn't have time to do it for the last X months.
Basically being a TV writer is like working in video games; there's another person right behind you who would love to have your job because of the perceived glamor so terrible conditions persist.
Hollywood writers are glamorous? Hilarious. I've lived most of my life in SoCal and that's not really the general perception. Even movies about writers like Saving Mr. Banks don't depict it as a glamorous life, though Their Finest does a little bit.
Ultimately what drives screenwriters is their need to create. For those of us who do creative writing, be it for screen, dead tree, or compiler, there's this internal need for the work to make itself manifest. We're just the conduit for the story. That's why people put up with the conditions in the writers room, it's all for the story.
Basically being a TV writer is like working in video games; there's another person right behind you who would love to have your job because of the perceived glamor so terrible conditions persist.