The Goblin Army event is a Ravencrest-specific event in Path of Terraria.
The Town Scout only appears in Ravencrest, and only one scout can be active at a time.
If you have not completed the Goblin Army event yet, the scout can naturally appear in Ravencrest. After you have already defeated the event, the scout becomes much rarer.
During the Wizard Start Quest, the scout is effectively forced to spawn until you kill it, so that quest cannot be blocked by normal spawn chance.
If you defeat the Town Scout in Ravencrest, it starts the Goblin Army event there.
If you let the Goblin live the goblin will report that the town is guarded by a warrior and not to attack the town.
More on the Goblin Army event can be found here. https://terraria.wiki.gg/wiki/Goblin_Army
Any information not explicitly changed by the mod still follows vanilla Goblin Army behavior.