The nuclear submarine fulfills this need. Her stealth allows non-provocative, unobserved access to all ocean areas. Her nuclear propulsion permits high transit speeds for extended periods without the need to refuel. Her small, highly trained crew can operate, maintain, fight, and repair her while on station. And, her high state of readiness allows rapid surge capabilities when crisis looms.
Submarines can get to the fight quickly, unobserved, and often unopposed.
Submarines can reposition as needed, without staged logistics. Submarines can remain on station for extended periods, providing intelligence, indications and warning, and reconnaissance data. Submarines can conduct precision land-attack strikes. In a real sense, like the frigates of Nelson's era or the cruisers of the age of steam, submarines are key elements of naval maneuver, able to roam the oceans and take the fight to the enemy. Of greater value, submarines can accomplish this with an impunity unknown to their historic forebears.