SetStartLocPrio

This function is used in the config() part of the map script.

I'm wondering what SetStartLocPrio and SetStartLocPrioCount are used for.

Those are for setting the players start location if there are random start locations in the map.

So why does the world editor generate that code if the starting location is defined...

I said it's for random starting locations. Ofc the location is defined. But if there are random starting locations you just define the possible ones in the WE. While map initialization that function picks one of these locations for every PLAYING player (just like in melee games).