1) Email Performance, Email Link Performance
2) Monthly and case by case
3) I use them to analyze campaign success (% opened, % clicked) to understand what messages are interesting and what seems unpopular. Also, I make an email performance graph that shows how open rates and click rates change through a year. I wish Marketo delivered such a graph as an extension to Email Performance report so I don't have to make it manually in Excel 🙂
4) Myself, Email Communications team, marketing management