Everyone knows the Ontological Argument, far fewer about what Anselm does with it. Having proved to his satisfaction that if we cannot think something better than X, that X is true of God, he points out that X = the supreme good. Other divine attributes follow by conjunction: is the supreme good better with _____ or with non-_____? If with _____, then it is clearly better to have it with no limitation. The blanks then get filled with power (and then omnipotence) knowledge (and then omniscience) blessedness (and then intrinsic and utterly non-contingent blessedness) self existence (and so simplicity, eternity, unity…) etc.
Even if one proved God’s existence another way, it’s hard to beat the fertility of viewing God as that-than-which.