When and why pyramids of Giza in Egypt built?

The Pyramid of Khufu also known as The Pyramid of Cheops [his Greek name] or The Great Pyramid of Giza was built by the fourth dynasty pharaoh Khufu over a period of about 20 years as his tomb around 2550 B.C. Hemiunu or Haman is believed to be the architect of the pyramid as archaeologists found mentions of Hemiunu with titles roughly translated as 'Master of Works' and 'Vizier' the highest official to serve the pharaoh Khufu. His mastaba [tomb] is close by.