Pyramids and tombs were built as a way to protect the body of an individual after they were dead (usually only important people like kings and pharaohs). Plus, these buildings were loaded with gold and riches of all kinds so the dead person could take them into the afterlife with them (they also built traps and mazes in them to throw off thieves).
In summary: they housed the bodies of kings and important people to send them to the afterlife in peace.
