Thanks Henning for your response. I'll add to the tracker.
So a scenario that I would use it for: Say I need students to book in an appointment. When a student has booked themselves in for a slot they have 'completed' the activity. This opens up quite a few possibilities e.g. It allows me to then target students who have not completed it (booked an appointment) by sending out an automated reminder email through the re-engagement plugin. Or I can send out an automated email to users who have completed it with information that they need for the meeting or class.
It allows me to download a spreadsheet showing who has and hasn't completed the scheduler activity by looking at the Activity Completion report. It also allows me to restrict access to other activities on the page e.g. I don't allow students to be able to view a certain document until they have booked an appointment.
Another useful completion criterion could be that the activity is 'complete' when a student has been marked as seen.
Thanks, Catherine