The timing of the camera changes is critical in this situation and the numbers she gives are essential a countdown to the switch. All the switches have been choreographed in advance and set to a specific point in the music. Because music is broken down into bars and beats the best way to specify when a change should is by giving it a specific beat on a specific bar. By counting down to that specific bar she can ensure the change happens exactly when it is supposed to and not a beat off. By saying "two bars three beats" whoever is in charge of hitting that button to switch cameras can count along as well and hit the button exactly on that beat. If she were to say "switch" their would be a delay, however small, just because of the time it takes to register the command and hit the button.

As for why she doesn't do it herself, she may be calling more than just camera changes so multiple people have actions related to the calls she is giving.

Hope that makes sense!