I sure don’t know. I think it must have been sacred or otherwise very special because it was built in a ridiculous location, even for the Incas.
A few years back (probably before 2005, but anyway) I read that each of the houses in Machu Picciu had firewood piled in front of the door. That meant that the houses were vacant, that the population left either en masse or at least during a short period of time. That’s part of the delightful mystery that is that site.