1it kills without wrecking the room
The blast leaves terrain untouched, which is the whole difference between this and wiring up explosives. You can line a hallway, an arena floor, or the approach to an NPC house and not have to rebuild anything afterward. It sits dormant until something walks onto it or a wire tells it to go off, so it is also fine to place and forget.
the polite way to rob a town NPC
Several items only drop from friendly NPCs, and killing them normally is fiddly. A mine does it cleanly, leaves the floor intact, and leaves the drop sitting where you can pick it up rather than at the bottom of a crater.
sweeping for someone else's
Minions cannot be hurt but they can still set mines off. Summon a walking minion some distance away and let it stroll back toward you and it will clear the path for free. A sentry placed directly on one does nothing, so do not use those to test ground. In multiplayer the mines will kill players who have PvP switched off, and Echo Coating makes them invisible, which is either the best or the worst thing about them depending on which side you are on.