People using it wrong. It definitely should not be that popular. 95% of times when I see it I consider it a tech debt.

You should be using it in rare cases when you want to verify very complex code that needs to be working with strict requirements (like calling order is specified, or some calls cannot be made during execution of the method).

Usually it is used for pointless unit tests of simple intermediary layers to test if call is delegated correctly to deeper layer. Those tests usually have negative value (they test very little but make any modification much harder).