If you find someone’s copy of a book that was an important influence on them, then by reading their marginal notes you can learn something about their work or life. We probably won’t learn much new about Shakespeare from reading Milton’s comments, but we can potentially learn a lot about Milton.

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.