Timelines matter. They help us know what information was probably known and shared by the author and what would have been unknown.
As an example, Paul was writing his letters decades before the Gospels were written. That matters because assuming Paul was using the Gospels as scriptures is making Paul responsible for stories he might not have heard.
All of the assumptions about dates are just that, assumptions. Scholars look for clues in the writing and then try to match them up with history periods.
As an example, going back to Paul again, he did not write about the destruction of the second temple because he was dead before that destruction happened.