As far as i am aware its a back up battery for various keep alive memories,eg transmision it is usually listed on the scan tool as the auxilary battery,which has a switch over relay to keep it charged,it also is a back up for the electronic brake system.
I may be wrong about its function but i,m sure someone will correct me if so.
I know for sure on much bigger mercs the additional batteries are for a RPS redundant power supply which is a back up for the electric power steering in case the main power supply goes down whilst driving.