I'm convinced that pyramids were built more as bunkers than as tombs. Sure they were used as such, but consider their features and utility.
High-ground gives archers range, and long line of sight. Horses couldn't traverse the steep surfaces, and foot solders would have to fight uphill. Traps and secret passages make navigation a daunting task. Stone construction prevents arson and render any sort of siege weapons useless. Food and novelties ensure a long stay is possible.
Rulers of these ancient cultures often had invading armies and their own subjects to to hide from. Pyramids seem like the most practical solution to this problem.
The pyramids of Giza were power plants.
Read the book, the Giza power plant by Christopher dunn. He makes an extremely compelling case that these were ancient high tech machines.