This is the last credit in this category warning message

In the submission system, I've noticed a new-ish warning message that's got me confused.  If there is only one credit listed in cast or self (I haven't tested other categories), and you try to delete it, you get a "This is the last credit in this category" warning.

What is the purpose of this warning?  Why should deleting the last credit in a category be any different from deleting the second to last, or the third to last?  Deleting a credit is deleting a credit.  Who cares if it's the last one?  Does deleting both credits from a listing that has two credits result in a similar warning?