We've said it more than one place on this site: conflict is the basis of plot. Without conflict, there's no story to tell because there would be no change or growth.

Robert Olen Butler puts it in terms of yearning: "Desire is the driving force behind plot. the character yearns, the character does something in pursuit of that yearning, and some force or other will block the attempt to fulfill that yearning." That's plot in a nutshell.

You might also think of conflict as a question your story sets out to answer. What is the central question of your book? Your conflict or question might be half-formed when you write the first draft, but before embarking on a revision, it's essential that you define the conflict of your story for yourself. And once you've defined it for yourself, then it's much easier to know which parts of your book or story to cut and which to expand.