The simulation driver was originally created to build and demonstrate HMI screens without the need for a PLC. If you write to an address in the simulation driver, it remembers that value for displaying back.
For instance, if you use a DigitalPanelMeter with the SimulationDriver, then put MyAddress in both PLCAddressValue and PLCAddressKeypad, you can click on the meter, enter a value and it will then show on the meter.