There are two good reasons to use the emergency brake, neither of which have anything to do with preventing the car from moving – the first is that frequent use will prevent it from becoming “stuck” and the other is that it will help keep your brakes adjusted.
The second reason for using it is that on the vast majority of vehicles, the emergency brake is used to activate the self-adjusting feature employed in most rear brakes, especially when drum brakes are involved. The frequent use of this system and that feature will ensure the brakes are kept up to snuff.