This lengthy quote by Herman Bavinck is a fantastic explanation of the importance of general revelation from a Reformed perspective.
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 moderns, to assume a radical break between the power of the good and the power of nature. Ethos and [phusis] 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 he world and the kingdom of God, of the natural order and the moral order, of creation and re-creation, of [phusis] 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, 4 vols. [Grand Rapids, Baker Academic, 2003], 322).