Just want to say that backstory exposition (which is usually done via flashback, but for me, is preferably done through the character or other characters revealing it themselves) is extremely important to character depth. It explains why the character is the way they are and both makes them more realistic and more relatable, since it enables you to understand why they feel things and also know more deeply how they feel.
Except that in most cases it boils down to "Here is average Joe Schmoe, doing his average routine, when suddenly Darth Vader shows up and throws his girl friend Hottie Hots off a bridge and into a burning building. Now you understand why Joe Schmoe is a heavy drinker and dirty skirt chaser." or "Here is Abby Normal. Abby and his older long white haired brother got into a slap fight which concluded with his brother shouting how much he hates him and how he wishes that Abby had never been born and so on. And now we return to watching them diving everywhere shooting Uzis at each other in a cathedral with pigeons magically popping up out of nowhere and flying off dramatically."
The problem with flashbacks is that they either assume that the main character is your basic muggle until whatever happens during the flashback happens then he's Shonen Superhero #8675309 or that whatever it was that turned them into a Shonen Superhero doesn't really make all that much sense to begin with. The other problem that occurs is why we are seeing the flashback to begin with? Does it have anything relevant to do with the immediate conflict? Is the situation the character is currently in related in anyway to what's depicted in the flashback? Is there even a need for an overly long and drawn out flashback when but an off-hand remark could manage in that situation?
The tl;dr is that flashbacks do not always equal character development or even when they do it's usually done in a horribly overwrought and needlessly convoluted way. Especially when you remember that in order for flashbacks to work, you're creating brand new characters for everyone involved, many of which do not see development beyond said flashback (unless you do a flashback of a flashback which is what stuff like the Star Wars New Trilogy are).