The uniqueness of the seal impression announced in December lies in its being a rare solid piece of evidence that Jerusalem housed a major Judahite royal administration. Almost all other seals and bullae from the Judahite era had been illegally dug up and sold by antiquities thieves, rendering them valueless to archaeologists because of their uncertain provenance and questionable authenticity.Although, as the article observes, the iconography of the seal doesn't offer any great support to the biblical picture of Hezekiah as an anti-idolatry reformer.
Background here and links.