...I'm not sure exactly why, but in the station when you pull the trucks in, you have to plug the truck in at a power source in the ceiling...
Fire apparatus typically have small electronics chargers inside them, such as for flashlights, and the radios are typically wired so they're always on. These big diesel vehicles have a tremendous amount of battery capacity, but you never know how long the vehicle is going to be sitting, so whenever possible, they're connected to a shoreline to keep them charged up and ready to go. They also may have block and battery warmers which work off the shoreline as well.