It's a proxy for both the movie quality and for where the cinema winds are blowin.
It also adds context to better understand what's at stake for movies based on distributor, producer and genres. When a horror movie costs 2M and a Marvel film 200M, "20M box office" by your favorite director can mean two very different things for their future.
Leaving aside that it's an example and any other movie could be there: Marvel movies have collected multiple awards including 2 Hugo awards, 4 Academy Awards and 2 Grammys. I think you'll have a tough time finding any measure of quality that at least one of their movies doesn't fulfill.
What does box office performance have to do with quality? Already looks like the subreddit has the wrong focus.