“... but historical records of when the pyramids were built, for what purpose...”
What historical records cite when they were built?
Dating of the pyramids is based on Kings Lists - the most used is a known fake, and the second most used seems to be some sort of copy of the first. What physical record do you cite?
What historical record cites purpose of the pyramids at Giza? There are paintings on some pyramid walls like the Pyramid of Djoser (which may turn out to be newer than the Giza pyramids) - but none on the Giza pyramids.
The same historic resources that cite:
“Nor were slaves used to build them - those that worked on the complex were well paid, fed, and cared for - it was an honor to work on them.”