Why might a writer break up the sequence of the plot?

Writers often flash back or jump forward, or use parallel plot lines. Depending on the story, it may be too much information--or too boring-- to tell a story in chronological order. It may be more interesting, for example, to start in the middle, so to speak, then fill in events from earlier while telling the story forward.

An example: Saving Private Ryan begins in military cemetery with an old man. He flashes back to just before the soldiers land on the beach, and it soon moves into a story about finding Ryan. Flashbacks are used throughout for many of the primary characters. Captain Miller, Ryan, and a few of the other main characters all go backward to tell something about their pasts.