It is narrated from Hazrat Abdullah bin Abbas (may Allah be pleased with them) that the first revelation of the Holy Quran was the complete Quran at once in the sky of the world, and the second revelation was gradually upon the Holy Prophet ﷺ.
The scholars have written about the wisdom of revealing it all at once in the sky of the world before revealing it to the Holy Prophet ﷺ that it was intended to clarify the loftiness of the glory and greatness of the Holy Quran and to tell the angels of that place that this is the last book of Allah Almighty which is to be revealed for the people of the earth.
And by revealing it twice, it is also intended to show that this book is above all doubt and suspicion, and it is preserved in two other places besides the blessed heart of the Holy Prophet ﷺ, one in the Preserved Tablet and the other in Bait-ul-Izzat.
Reference: Tuhfa Hufaz