Think about a book that is all action, you think it might be exciting but really it would be tiresome. EPISODES exist for a purpose outside of promoting plot or advancing action. To illustrate this, I quote from a book I read recently that I really, really liked and reviewed here on Goodreads: Mississippi Cotton by Paul H. Yarbrough (WiDo Publishing, 2011).
The following paragraph is just part of the EPISODE which involves a man stopping by the farm at breakfast. It doesn't move the plot along, there is no real action; even the character mentioned, Earl Hightower, is not a main character. But it creates background, illuminates character-- the Southern character-- and sets a tone. EPISODES enrich and add depth and interest to the story. Mastering them are a crucial part of learning the craft of writing.
Earl put his brown hat in the chair next to him. In his work clothes, he looked tanned and strong—a real cotton farmer. His blue cotton shirt sleeves rolled up revealed big hairy forearms, with hard-looking muscle that came from farm work. He had a gentle way about him, but a mannerism that made you know he was definitely no softy. One of his big hands swept around the cup, not using the crook, and took a big swallow. Black. No sissy coffee for Earl Hightower.
(This post has been inspired by and in some instances, directly quoted from A Handbook to Literature, 8th Edition, by William Harmon and C. Hugh Holman)