Flashbacks are a troubling and wonderful thing. They can provide essential insight into a character’s personality or motivations. They can develop dramatic tension by making the reader privy to events in that past the character isn’t aware of. They can set up thematic fulfillment with scene changes.
They can also be confusing, or a crutch for weak characterization or plotting. They can mess up the flow of a story, throw off pacing, and stick out like a sore thumb.