Why is it necessary to control the call count wait in a function in mock objects?

Question

PHP

Good afternoon!
I'm currently learning unit testing. Everything is clear, except for one thing: I don’t understand why unit tests control the number of function calls.
First, a mock object is created.
$mock->expects($this->once()) or other number of calls.
Why is such a design needed? Is it possible to see a practical example of where exactly it needs to be called once and why it is needed?
Thank you!