I'd wager most people prefer movies with actors they know (and like). For example if Morgan Freeman is in a movie I'm already thinking there's at least one decent actor in there, so the likelihood of me watching it goes up.

And of course you know what you get usually. If Jim Carrey is in a movie, duh, you get slapstick humor most of the time. Samuel L. Jackson will probably play a cool guy and drop a few f-bombs. Chris Pratt? Probably an action movie with lighthearted humor. And so on.

It's not only about the actor, but each one has a kind of brand too. And if the actor is expensive the production quality is usually decent.

While I don't think it's as straight forward calculation as Norgur thinks, you're forgetting that star power has marketing reach beyond just name recognition on a poster. People want to hear from them. They give interviews, promote at events and give status to the movie: It will be featured more in media which in itself means that more people will hear about it (even if they don't choose the movie based on that name) which means they're more likely to consider seeing it at a later date as they recognize it.

The "A-list actor" isn't much of a thing nowadays - it's all much more about franchises - but star power used to be a real box office draw. People would go to see a film just because it was the new Johnny Depp film, for instance, regardless of the genre/plot/style/quality.

There aren't many actors these days who have that kind of draw. Two that I can think of are Ryan Reynolds and Tom Cruise - both actors where you have a fairly good idea of what you'll get from a film they're in, even when you know nothing about it.

Franchises have definitely taken over. People will go to the cinema to see a film because it's a Marvel film, a Star Wars film, etc, regardless of who directed it, who it stars, and the quality of the film. Sequels and cinematic universes sell tickets in the way familiar faces used to.