A first act creates the context: the facts a reader needs to understand the story going forward. It contextualizes a lot of things including:
Who are we following and what’s their deal? What’s their archetype? what are their distinct traits? What do they want? Take a movie like THE VERDICT. At no point does Paul Newman say, “I’m an alcoholic sellout lawyer who must win a case or lose my soul,” but the script illustrates this as clearly as if it were stamped on his forehead.
People often worry about likability or relatability, but both of these take a backseat to clarity. It’s hard to emotionally invest in a character who you can’t even wrap your head around.
A script will set up the ways in which this setting differs from the common sense world we hold as normal. The social codes people live by. The technology levels. The rules of magic or sci-fi tech (if applicable). The kinds of things the people therein think are impossible or normal. (Example: This morning I rode my purple dragon to work and I heard a man talking about love. Love? What nonsense!).
Period scripts or scripts that are densely political share a lot of the expository needs of fantastical settings. If a script takes great pains to tell me that we’re a week away from the seven year anniversary of the coupe that enabled Colonel Santos to take over the town, it’s a safe bet that this information will be necessary to know going forward.
Finally, and perhaps most importantly, it creates a context of you for a reader. Is the writing clear? Is it interesting? Does it seem like the writer prioritized communicating and telling a good story over and beyond their selfish need to express themselves and make money? I’ve often said a script can lose a reader on the first page (or even the first line). You keep them by providing a clear context to hang an understanding of what we’re rooting for. If it doesn’t, it’s dead in the water.
Contextualize quickly, efficiently, then move on
The script needs to get all this setup done in a brief amount of time. By about page 25, the audience is done learning. The time for setup is over, we want to see what fun stuff the author can DO with all that setup.