Stuff that they don t need, but still needs to be dealt with. Not necessarily read. The writer does have a need, not to get lost in that submission pile. It used to be true, and probably sometimes still is, that if you wrote a lot of letters to comic book titles, your name would get known. Submissions might not be high on anybody s list to read.
Editors normally have to get out one more title than they comfortably can handle.
The unsolicited scripts are a factor they don t have time to really look at. By the editors. Often the writers and artists. You are writing about their work. They re going to want to know what you thought.