Why would the ancient Egyptians devote so much time and resources on the pyramids, if they were just tombs?

 So, when humans were first developing agriculture, they had a bit of a food surplus and, as a result a population boom.

They also didn’t need nearly as many people to keep the fields growing and everyone fed. So you end up with a massive surplus of manpower that has nothing to do for a good chunk of the time.

So, what do you do to avoid them just laying around and getting into trouble. Well, you come up with some projects.

Of course the first ones are going to be things that support agriculture, Canals, Dams and what not.

Of course, your success might make other people interested in what you have, so you may then spend some time building fortifications.

But, at a certain point, you still have a massive population, far bigger than what you need for agriculture. And it’s even worse because of your irrigation canals and the walls that prevent those weirdos next door from stealing from you.

So, you come up with other big projects. And Viola

You see it in other civilizations too. Anywhere you get a lot of surplus manpower with not a lot to do you get stuff like this.

Great Pyramid of Chichen Itza

Nan Madol (Seriously, look it up, fascinating place)

Great Wall of China.

In a lot of cases, these projects didn’t serve any rational or real purpose. Their primary role was to put the population to work and to give them a way to “Earn their keep”.

So it is basically a natural part of the development of Human civilization.