Seeing when your students are online.
This is useful because... You could tell them to log on the next day and if they don't.. Dun dun duun dun. Detention.
Yup. And ask them if they are ok and would they like a chance to make up the assignment. Though, personally I'm not in favor of anyone being able to see when anyone else is online here, unless that person elects to update online/offline messages in their editable profile section.