It's probably to deal with surge current if you get a charge imbalance. Say you're sitting there with motor off and running from house battery for a few hours, drain the house battery to, say, 11.5v; when you start the motor and the charging relay opens there will be a significant flow of current from the start battery to the house battery as the system tries to hit a happy equilibrium of voltage. Fuses are funny things in terms of the rating; their momentary rating is usually way, way over what's stamped on the side, likewise cable.
10AWG is likely fine for 20-30A sustained, especially if it's a very short run of cable. I'd consider going up to maybe 8AWG if it's a bit longer.
The fuse is mostly there to prevent a fire if you short a cable out. If you get a dead short the battery will happily dump about 500A into that cable and pop a 50A fuse before you can say "oops".