I have often thought it would be nice to time travel and have a look to see what was going on in the eras that science tells us these things were built. I would love to pop back to 5000 BC and find the sphinx already built and considered ancient! See people building Avebury without the use of pulleys? Simply some crazy control of gravitational fields that we don't remember anymore! Maybe Von Daniken is right and little green men will be building them! Who knows?! But without that time machine (and perhaps even with it) we will never know what these things were for. Maybe we would get there and the people would be building these monuments without really understanding why. Simply because there were older things (like woodhenge) there and they knew that at a certain point in history they needed to build something bigger and better.
But would I want to know? I think most of the appeal is not knowing. They are mysterious and beautiful and they can mean whatever we want them to mean and thats part of the attraction for me.
:)