Why is rainbow invented?

I believe the Bible, therefore, I believe that the answer to your question is in the Bible. The meaning of the rainbow is clearly stated there. Here is what it says. "I set My rainbow in the cloud, and it shall be for the sign of the covenant between Me and the earth. It shall be, when I bring a cloud over the earth, that the rainbow shall be seen in the cloud; and I will remember My covenant which is between Me and you and every living creature of all flesh; the waters shall never again become a flood to destroy all flesh. The rainbow shall be in the cloud, and I will look on it to remember the everlasting covenant between God and every living creature of all flesh that is on the earth.” And God said to Noah, "This is the sign of the covenant which I have established between Me and all flesh that is on the earth." Gen 9:9-13 But the traditional meaning of the rainbow has always been the Biblical one that God promised never to destroy the earth with water again. In Revelation 4:3 and 10:11 the rainbow is found very close to God because Revelation is a vivid picture of God’s faithfulness in history. He will keep His promises and nothing we can do will prevent that, anymore than we can interfere with the sun and the rain. We must understand that God will judge this world again, only this time with fire, not water. I hope that helped you. Have a great day!