Default event types don't have a What field (and have non-compulsory Summary further down instead), whereas custom events do have the compulsory What field (towards the top and no Summary).
It's very confusing for users to have to create/edit calendar entries differently depending on the Event Type. We don't differentiate Event Types as custom or default internally - we just want the calendar to work the same way for everyone/everything.
The What/Summary fields show as the “description” in the calendar view so they are helpful.
We would just prefer not to have them as different fields for each event type - and not be compulsory.