Campaign email performance to keep track of email performance on specific emails.
Most of the initiatives I run in Marketo span multiple programs, so a report like this is handy to watch what's going on with a initiative without having to hunt town each smart campaign. For some it's a weekly report, but right now I'm running a few daily for triggered emails to new leads.
Generally I'm the only one actually looking at the report, but I'd certainly use the data in the report to share with the salespeople affected by the emails.