The fuse is there to protect the cable in the case of short circuit.

0 gauge can run 150amps all day. But if there is a short circuit ie the insulation wears through and it grounds on the chassis, it'll flow infinite current. The wire heats up, melts the insulation, and causes it to catch fire, burning your car to the ground.

So we install a 140amp fuse. The starter wont draw this much, and if the cable does happen to short out, it'll pop the fuse instantly.

You have the fuse mounted with in 20cm of the battery, so that you only have 20cm of unprotected cable.