Podio API: item.delete

Hello,

I do not understand how the item.delete call in triggered.

I'm using the item.created and item.updated calls to monitor changes in a field (Tour Date).  When I add a new app item with the tour date entered, I get the trigger for it as expected. 

When I update the Tour Date to either a new date value or remove it (empty/null) i get the trigger call from the item.update trigger.

I do not understand what purpose the item.delete trigger is used for.  I was assuming it was for when Tour Date is set to empty/null, but that is not the case.

Please advise what purpose the item.delete trigger/call is used for and under what trigger/event to occur.