The most important rationale for reading, and reading wide is development — widening and deepening — of grammar or syntax, and exposure to different styles of writing — polemic, anger, satire, apologetics etc.
Some novels are great but difficult to translate into good movies e.g. Anne of Green Gables by Montgomery, L. M., The Scarlet Letter by Nathaniel Hawthorne etc..
Others are great and easy to translate into great movies such as The Count of Monte Cristo by Alexandre Dumas (the first half of the movie was great; the second half, a mess an insult to the novel).
There must be a lesson in these differences between great novels for an aspiring screen writer.