It is there to protect the wire that runs from the main power buss to the battery master switch. In the first position, the master switch connects the battery to the main buss, but the path from the main buss to the alternator field is not yet connected, and neither is the field circuit breaker.
The fusible link is an inexpensive fuse to protect that energized wire should it (unlikely) be shorted to ground through a chaffed wire or bad switch. You will note that all other wires from the main buss are fused.
Don