As a reader, there was nothing I hated more in a book than having a main character vomit their entire life history in an attempt to explain some previous reaction or inaction. It's a classic case of violating the SHOW don't TELL mantra that writers all chant to themselves in their darkened basements. It's also just sucky.
As a writer, it's important that your characters have some sort of history to make them three dimensional. It informs how the characters would react in a given situation and can allow you to leave a trail of breadcrumbs to entice the reader to follow.
Take a classic example from the movies: Darth Vader. Using a nice little bit of misdirection with Obi-Wan Kenobi - telling Luke his father died - the reveal in The Empire Strikes Back that Darth Vader was in fact Luke's father caused the world to choke on its popcorn.
From recent internationally best selling books you have the character of Severus Snape. Harry Potter spends almost the entire run of books hating Snape. Harry, being the likeable protagonist, has the sympathy of the readers so they in turn hate Snape. However, Rowling did a great job of teasing that there was so much more to the story that you weren't being shown. For example, Snape had Dumbledore's utmost respect and trust. This never made sense to Harry and less so when Snape killed Dumbledore.
Now, Lucas went back and showed the world what happened before Star Wars in three abysmal prequels. When watched in order now, you take away all the mystery of Darth Vader.
If Rowling had made Dumbledore explain to Harry in her first book why he trusted Snape, that would've ruined pretty much the rest of the entire story, since Snape was one of the most interesting characters.
So, it's important for a writer to know how their character is to be used in the story but not a good idea to explain it all to the reader in one paragraph. The beauty of good writing is being able to slowly peel the layers back, revealing another level to a character as the story progresses. And with each layer, you will lead the reader further on the trail of breadcrumbs until they are lost in the story.