What does an SSGN add to a navy's conventional deterrent?

Why are SSGNs useful, and what explains the interest in them?

They can launch a large conventional strike from an unexpected undersea position, offering carrier-scale opening firepower without revealing the launch platform in advance.

A large cruise-missile magazine allows an SSGN to deliver a sudden strike from a location the opponent did not expect. The opening salvo is compared with the striking weight of a carrier, which explains the attraction for a medium-sized power contributing to NATO. The British concept described here combined continuous strategic deterrence with a carrier battle group and cruise-missile submarines kept at sea for conventional deterrence. Whether that required one or two SSGNs on patrol remained a matter of debate. The proposal competed with the political preference for maintaining the British Army of the Rhine.