on Fiction University:

A character’s history is important, but not enough to bog down your entire story to hear it.

Along with adverbs and telling, I think backstory completes the unholy trinity of writing. So much so that agent and writing guru Donald Maass advises writers to cut any backstory in the first 50 pages.

But backstory does have its uses, and sometimes, it’s critical to know that history.

Even if it’s not critical for the reader to know it.

In some genres it’s more of an issue. Fantasy, science fiction, historical—any genre where the past and the history of that past strongly affects the current plot and the motivations of the characters. Doubly so if the antagonist is the one with the past that’s come back to haunt someone, since you don’t always see the antagonist’s POV.

One of my WIPs has a major event that happened decades ago, but this event is the trigger for all the present-day plot events in the story. I knew basically what had happened in the past, but I focused more on what my protagonists were doing/uncovering and chose what parts of the antagonist’s plot to use based on them, not what had actually happened.
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By the time I was done, I wasn’t happy. The mystery part wasn’t as strong as I knew it could be, because I hadn’t spent enough time on the backstory. If you looked too closely at the plot, things didn’t quite line up, and questions were left hanging.