That is best and most honest answer there is about why Stonehenge was built - we really do not know.
All else is speculation and worse it is speculation into which very modern views of many things are inserted into the minds of those who built it.
There are NO ancient “writings” connected to or with Stonehenge that were left by it’s builders. All that ANY archeologist can do is speculate.
On the bottom of each stone has an ancient inscription.
It say’s MADE IN CHINA