What is the virtue of saying "Lā ʾilāha ʾillā-llāh"?

The phrase "La ilaha illa Allah" (There is no god but Allah) is the best form of remembrance after the Quran. It is the key to Islam, its pillar, and one of its fundamental tenets. Many virtues have been reported concerning it, including that it saves its utterer from Hellfire, makes Hellfire forbidden for him, and is a cause for his entry into Paradise through any of its gates. It is the heaviest thing in a servant's balance on the Day of Resurrection; with it, the scrolls [of deeds] become light, and nothing can outweigh it.