At some point it was probably considered artful for a movie poster to show part of a face with one eye. Maybe, I dunno, I'm not a real scientist. All I know is after 2000, it became a studio imperative for all movie posters to show the entire movie cast all squished together, so slicing faces in half just started to make sense. 

Sometimes people's obscured faces obscure other people's faces.

Sometimes God holding a gun obscures a face. 

Scale is not important when obscuring faces. The faces can be all different sizes. Who cares. 

It's actually rare for comedies to obscure faces but they're catching on. 

Human or not, if it has a face, it should be obscured. Fuck it. 

Here's how the face-obscuring hierarchy works: Black people are obscured by white people. Paintings of white people are obscured by white glass. 

Sometimes people obscure their own faces. 

And sometimes the person's face isn't symbolic enough of the person, so that person will obscure their face with something that represents them even better than their actual face.