(by chris the cynic)
More than two and a half thousand years after the stuff was composed, sometimes we only know who someone was because someone decided to write a note saying, “Totes refering to person X” or sometimes we would know who they were, but not their name if not for the note saying, “By the way, her name was totally Y.”
As time went on some manuscripts had margins larger than the actual content of the work theoretically being written/printed. New marginalia has been produced constantly, and writing it is a practice that continues to this day, as anyone who has bought textbooks used can tell you.
We call it scholia, and while it may lead us in the wrong direction if the person writing it was horrendously wrong, with the passage of time sometimes all we have is what someone scrawled in the margin.
Also, sometimes the suggestions of the scrawling scholiast’s notes make for much more interesting stories than the book itself or, indeed, the truth (see: The Philadelphia Experiment.)
[As a reminder, open thread prompts are meant to inspire conversation, not stifle it. Have no fear of going off topic for there is no off topic here.]