Convenience for one.
Your key switch probably cannot handle hundreds of amps (or many volts either). So you use the key switch low power/voltage signal plus your small 12 volt battery to power the coil on the contactor and close the circuit on the large battery pack. The Allbright SW200 has a 12v coil.
They have some wiring diagrams for the controller, you are going to have a separate 72 to 12v dc-dc converter and 12v battery for accessories? I'm just the messenger
FYI, the
alltrax documents for the axe seem to be using a 72v contactor and accessory relay. It gets rid of the need for a 12v battery, but you expose your key switch to 72v, so maybe that is a reason to have an interlock.
i.e. these two mixed together, the vehicle key is in the second one
http://www.alltraxinc.com/files/Doc1...e-wire-dia.pdf
http://www.alltraxinc.com/files/Doc1...y-wire-dia.pdf
I would probably have done it differently.