What is the wisdom behind delaying the revelation of "from the dawn" in His, the Almighty's, saying: "and eat and drink until the white thread becomes distinct to you from the black thread"? And was "from the dawn" explanatory or a completion of the verse?

Allah Almighty permitted the fasting person to eat and drink at night until the dawn became clear. The 'white thread' refers to the day, and the 'black thread' refers to the night. Some companions misunderstood the verse; one of them would place two threads, one white and one black, under his pillow and continue eating until he could distinguish one from the other. The Prophet, peace be upon him, corrected this understanding and explained to them that the white thread meant the whiteness of the day and the black thread meant the blackness of the night. This was not a dispraise or an attribution of ignorance, but rather the Prophet, peace be upon him, was clarifying the meaning for them. There is no issue with some companions of the Prophet, peace be upon him, making mistakes in understanding Allah's intention from a verse, for they are not infallible. As for the delay in the revelation of "from dawn," it was not a delay in clarification beyond the time of need, but rather a clarification of the verse for some companions for whom its understanding had become obscure.