Why do we have and use musical periods?

An oxbridge music interview won't require a written out answer, but it's good to get used to following your thought patterns through. categorising widespread media is a practical way of enabling conversation about themto show a progression from 'primitive' to 'sophisticated'different styles lend themselves to being grouped and compared (why compare? to rank?) (e.g. continental and English polyphony)as a record of creative outputwhat makes it into a history? who decides the criterea?they are mostly retrospective, so categories reflect adgenda of those categorising - this can be useful to later understand what music history was being used for