What is the purpose of superlaser-equipped Super Star Destroyers in the Tarkin Doctrine (Legends/Canon)

Given that the Tarkin Doctrine is based on the idea of overwhelming force and fear of force, with the threat of destroying planets as the ultimate punishment for harboring Rebels which the Death Star was built for, what's the original idea of equipping SSDs with superlasers in the Tarkin Doctrine? Are they meant to replace the Death Star (since a fleet is much cheaper than a moon sized battle station) or are they for a different purpose?