I "grew up" as a writer in the era of NO BACKSTORY ALLOWED. I was given examples of stories and books that had zero backstory and engaged readers completely. So I worked hard to eliminate any pesky references to the past--whether summarized as backstory (background of the story) or presented as flashbacks in scene.
I got published, and all was well in my writing life sans backstory for many years. Flash forward to my MFA experience and advisers who began to cure me of my antagonistic attitude towards stalling out scene with flashback or inserting large swaths of the past as summary. These writers hinted that backstory was important, even as an explanation of character motive. Why people do what they do was becoming more interesting to readers than what they did.