Ok, lets look at the standard things that people build really big structures for. Not in thousands of years ago, but now, so we have no doubts about what's happening.

1. Killing each other (fortification, aircraft carriers)

2. Selling stuff to each other ( malls etc)

3. Worshipping something/someone( catherdrals, sports stadium)

4. Transporting stuff (roads, bridges, dams etc)

Now, lets assume that the pyramids were built by people, and their motivations haven't changed too much. Which explanation fits best?