What are torpedoes common for?

Answer 1

There are two ways to answer this question: "What are torpedoes commonly used for?" and, "What are torpedoes commonly deployed on?"

Torpedoes are anti-ship and anti-submarines weapons. In both cases, their purpose is to sink the target ship/submarine. They are very powerful weapons, and a single hit from a torpedo has a high probability of either crippling or sinking virtually all ships and submarines currently in existence.

Torpedoes are deployed on aircraft, helicopters, naval ships, and submarines. Cruisers and Destroyers which are equipped with anti-submarine sensors and equipment will typically mount torpedo launchers. Destroyers, Frigates, and smaller navy ships which do not have anti-submarine equipment will sometimes carry torpedoes for use in an anti-ship role, though this is now a lessor option on newer such ships, as the anti-ship missile has mostly replaced the torpedo as the primary anti-ship weapon for surface vessels.

Submarines (so far) always have torpedoes, as it is (usually) their primary anti-ship and only anti-submarine weapon.

Helicopters and aircraft which are designed for the anti-submarine role always carry torpedoes. Helos and planes which are designed for an anti-ship role carry air-to-surface missiles instead.