Since when and why did the Egyptian kings wear the double crown? What did she mean?

At the end of the fourth millennium BC. e. in the Nile Valley two large kingdoms were formed (Lower Egypt and Upper Egypt). The king of Lower Egypt wore a red crown, and the king of Upper Egypt wore a white crown. Around 3000 BC e. the ruler of the southern kingdom defeated his northern neighbor. He began to wear a double crown – white, inserted into a red one, which meant the unification of the country into one state.