The testimony of John Robins is what teachers like to call “primary sources”. It’s a document prepared by someone contemporary to an event, and even if the narrator is unreliable, we learn much more from it than from, say, something that was written long after the fact.
In the case of John Robins’ testimony regarding events at Lexington and Concord, the events were still fresh in his mind, having happened only a few days earlier. And because it derived from an official proceeding, that document was reasonably well-preserved and serves as a fascinating window into one of the most important events of our history.
And as for Robins himself, he was a relatively common man who happened to be part of the militia that day and might have been otherwise forgotten.