Why ballistic-missile submarines suit a second strike

Are ballistic-missile submarines primarily first-strike or second-strike weapons?

They are generally second-strike assets because remaining hidden makes them difficult to eliminate before they can retaliate.

Ballistic-missile submarines are difficult to locate before they launch, which makes them valuable as a surviving retaliatory force. Their weapons are intended for broad strategic destruction rather than precision attack. That hidden endurance separates them from land-based ICBMs and air-delivered weapons, which the discussion treats as more typical first-strike systems. The platform's place in nuclear deterrence rests on survivability rather than missile performance alone.