HistoryArt

Crawling down into some narrow tunnels filled with potential threats isn't something most military forces are going to want to deal with. Raiders will likely loot the surface (for however many weeks that takes) and leave whoever is in the tunnels alone - assuming they even know where the entrances are. Occupying military forces are more likely to offer good terms of surrender than deal with the hassle of dragging everyone out.

Same basic idea as any fortification, but much more oriented towards civilian purposes - not as much use for denying enemies freedom of movement or otherwise tying down enemy armies, but great for preserving the lives of the people within against superior military force.