Nuclear weapons exist solely to deter other nuclear strikes. [causal]
In an era of nuclear weaponry, the industrialisation of war has proceeded to a point at which, as was mentioned earlier, the obsolescence of Clausewitz’s main doctrine has become apparent to everyone. The only point of holding nuclear weapons—apart from their possible symbolic value in world politics—is to deter others from using them.
Anthony Giddens, The Consequences of Moderni…, loc. 186