This is my own theory, and it only works IF we accept that the Great Pyramid of Khufu was not built by humans, but by aliens. Yes, that’s a big stretch, because the pyramid was (most likely) built by humans, but in the case that all these crazy conspiracy theorists are correct, then I could think of a different theory of why it was built.
I base my theory on the simplest answer of what the pyramid is. The simplest answer is usually the correct one. So, if you ask a child “what’s the Great Pyramid”, their answer would be: “it’s a big, big building”.
The only reason why an alien race would build a humongous building on Earth at a time when only huts existed, in my opinion, is that so it can be seen from space. There are not many spacious rooms inside the pyramid (so it wasn’t grain storage), it wasn’t a temple, and we already know that they weren’t tombs. What it is though is just that: a huge building, visible from space, with basic equipment.
Picture this: humans transition from hunter-gatherers to organized societies around the same time. When you have a young race evolving to become something more than animals, that could raise some alien eyebrows. And so they erect… a sign to all other alien races: “KEEP OFF”. Passerbys are much more likely to enter a wild field and claim it their own or just mess with it, than entering one that has a sign to keep off.
Building a large building visible from space is a much smarter way to accomplish this than simply putting a satellite on orbit. The satellite would need servicing, it can go bad at any time, and it would transmit at a frequency or digital format that another alien race might not understand. These problems don’t exist if you just erect a big-a$$ building though, one that it shows high mathematics in its various elements/ratios/location etc. Math is a universal language, and one that would be respected by another alien race that has already mastered interstellar travel.
On top of that, the pyramid shape and construction is earthquake-proof, so it can stay erected for thousands of years, as it has. I don’t know if the Bauval/Hancock theory that the pyramids are older is correct, but it’s of a little consequence if my theory is correct.
Now, as to the pyramids placement point to Sirius or not, I don’t know. But it is possible to fathom that the builder alien race did leave a clue about “please inquire at the XYZ starsystem for access”, just like one would potentially put a telephone number on a Keep Off sign. But I don’t think that’s necessary.
As to why any alien race would care to “protect” this new human race by leaving them alone to develop in peace and erecting big “keep off” signs, I think that protective (or even possibly ownership) tendencies exist in all biological creatures. I don’t think that these aliens would be much different than us in basic behaviors if they have a biological base in this universe like we do. As above, so below.
Of course, as I mentioned in the beginning, this theory makes sense only if aliens built the pyramids. Which probably they didn’t. But it’s nice to spend the afternoon theorizing, if they did.