Why where submarines used in wars?

The first submarines used in wars were not very mobile as no real engines had bee made for them, -So they had very short range and primarily used to plant bombs under or on the hull of enemy ships in harbour .

-By the time of WW1, submarines had electric motors for subsea propulsion and diesels for surface running (the diesels also recharged the huge battery banks) In this much more efficient form, submarines were used to attack enemy supply ships and thus starve the enemy of much needed supplies.

In WW2 and later, attack submarines were developed and these could often sink enemy warships.

In the latest type of nuclear 'boomers' , submarines can carry ICBM's or cruise missiles and could lurk anywhere in the world's oceans.