Why? We obviously read the notes and we didn't especially care for them before we found out they were Milton's, so why is that? This implies that we care more about a famous person writing dreck than an anonymous person writing fantastic insights.
Perhaps not in this case, but often marginal notes are incomprehensible without context about what else the writer of the notes was reading, thinking about, or working on. Once you know who wrote the notes you can often understand more of their content and relevance.