Seeking Guidance regarding Subject Matter of Stories

Hello, fellow writers (or those few watching me),
As I have experienced the many challenges presented in writing fanfiction from this site (writing my own and looking at others), I seem to have come upon a problem that I cannot understand: Is it okay to leave certain material in stories up to the readers' interpretation/imagination?
Two of the fanfiction I have written all start at a time where the main event, or even the essence of the story itself, starts. I have used this technique for I believe that matters within a story, such as backstory or development leading up to a certain point, are ignorable if the story's main focus is that one central event. However, I seem to be wrong in my belief and am seeking reconciliation regarding my perspective on this issue. I would greatly appreciate it if anybody were to be able to provide me with tips, and maybe examples, that will help me understand the value in backstories or "ignorable" story development and whether or not they should be used in stories focusing on one main event.
Thank you for taking the time to read this,
AcesRockz