Workout calendar items are a legacy feature used with a previous version of OnDeck. The system used to require that a workout calendar item exist in your site before you could take attendance using OnDeck for that day. This is no longer a requirement.
The only reason to create a workout calendar item now is to get the little red 'Workout' icon to display next to the item in your Calendar.
Can I change Roster Group Calendars to Workout Calendars for attendance tracking?
Workout Attendance Report
How do I enter an attendance record for a previous/past date in OnDeck?