IMHO: The most careful and thoughtful statements on the value of general revelation have come from the pen of Dutch reformed scholar Herman Bavinck. Here, for example, are few of his thoughts taken from the first volume of his magnum opus, Reformed Dogmatics:
“… To deny that natural religion and natural theology are sufficient and have an autonomous existence of their own is not in any way to do an injustice to the fact that from the creation, from nature and history, from the human heart and conscience, there comes divine speech to every human.
No one escapes the power of general revelation. Religion belongs to the essence of a human. The idea and existence of God, the spiritual independence and eternal destiny of the world, the moral world order and its ultimate triumph—all these are problems that never cease to engage the human mind. Metaphysical need cannot be suppressed. Philosophy perennially seeks to satisfy that need. It is general revelation that keeps that need alive. It keeps human beings from degrading themselves into animals. It binds them to a supersensible world. It maintains in them the awareness that they have been created in God’s image and can only find rest in God. General revelation preserves humankind in order that it can be found and healed by Christ and until it is. To that extent natural theology used to be correctly denominated a “preamble of faith,” a divine preparation and education for Christianity. General revelation is the foundation on which special revelation builds itself up.
Finally, the rich significance of general revelation comes out in the fact that it keeps nature and grace, creation and re-creation, the world of reality and the world of values, inseparably connected. Without general revelation, special revelation loses its connectedness with the whole cosmic existence and life. The link that unites the kingdom of nature and the kingdom of heaven then disappears. Those who, along with critical philosophy, deny general revelation exert themselves in vain when via the way of practical reason or of the imagination they try to recover what they have lost. They have then lost a support for their faith. In that case the religious life exists in detachment from and alongside of ordinary human existence. The image of God then becomes a “superadded gift” (donum superadditum). As in the case of the Socinians, religion becomes alien to human nature. Christianity becomes a sectarian phenomenon and is robbed of its catholicity. In a word, grace is then opposed to nature. In that case it is consistent, along with the ethical modems, to assume a radical break between the power of the good and the power of nature. Ethos [morality] and φύσις [nature], are then totally separated. The world of reality and the world of values have nothing to do with each other. In that scenario we at bottom face a revival of Parsism or Manichaeism. By contrast, general revelation maintains the unity of nature and grace, of the world and the kingdom of God, of the natural order and the moral order, of creation and re-creation, of φύσις and ethos, of virtue and happiness, of holiness and blessedness, and in all these things the unity of the divine being. It is one and the same God who in general revelation does not leave himself without a witness to anyone and who in special revelation makes himself known as a God of grace. Hence general and special revelation interact with each other. “God first sent forth nature as a teacher, intending also to send prophecy next, so that you, a disciple of nature, might more easily believe prophecy” (Tertullian). Nature precedes grace; grace perfects nature. Reason is perfected by faith, faith presupposes nature.”
—Herman Bavinck, Reformed Dogmatics: Prolegomena (Baker Academic, 2003), 1:321—322.