Call panel not in the cab -- so you must be talking about a "destination dispatch" system. With DD, you go to a call kiosk, and enter your desired destination floor, and the kiosk tells you which car to take. DD systems are done because it allows for much more sophisticated scheduling, and larger elevator lobbies with longer walk times, the net result being much better capacity, especially at peak traffic times.

In the cab you will typically have a button to take you back to the lobby in case you didn't get off on your desired floor and can't enter another call.

Modern destination dispatch systems are integrated with RF badge readers so that the elevator system can be programmed with your office floor, and so you don't need to go to the kiosk at all, just walk past it, and it will schedule your to a car to take you to your office floor.

The scheduler can group passengers together to get fewer stops, and longer express zones. Also, it makes double-deck elevators practical. Consider the elevators in the Willis Tower in Chicago. These are double-deck cabs, and it takes about three floors for the cab to accelerate to or decellerate from express speed and make a stop. Considering the number of people that system moves daily, it would be impossible to do without DD.