They create two holes - one at the starting location, one at the target destination - that are connected and allow allied units to travel through them in an instant. Tunnel-Diggers, on the other hand, are a small batallion of particularly brave (or particularly crazy, depending on the perspective) goblins that can bypass enemy fortifications and walls and emerge in the middle of the enemy base. Looters are available in the Goblin-Town Cave at level one. With this ability, every horde of Looters may summon another horde of Looters to their side for a medium duration and also steals resources from buildings. They may also use their special ability Lord of the Pillaging Hordes once the Great Goblin is recruited. Looters are weak melee units that steal resources every time they attack buildings. Goblin-Town provides acces to two units: Goblin Looters and Tunnel-Diggers. Instead, the warriors excel in raiding and looting enemy structures while the Loot Hoard offers specialized upgrades to strenghten your foothold.
Now keep in mind that we are still talking about goblins, so massive centralized investments into the infrastructure (in the form of Worker's or Settler's Tools) does not really fit the faction. Of the three orc realms, Goblin-Town focuses most on improving your economy and therefore offers fewer fighting units than the other two.