looks like i keep reading the cutcenes wrong lol. Im sorry and defenetly overlooked that part. So afterall the crytal towers purpose was to produce energy.
But while it did that, it was a symbol of prosperity, hence why it was open to public.

When Xande came back, he closed it. Turning into mostly only a power plant. They wanted more and more energy to feed the darkness, that when they destroyed pretty much the meracydians and caged bahamuth in dalamud, they tossed it in the skyes, producing more than before.

but as it was not enough, they overcharged ir somehow and created the blast, the earthquake of the 4th umbral era, where allagan empire literally sank into the ground at some points. Then amon decided to stop the towe from sinking doing that from before etc...

wich means CT wasn't built neither as a monument but yes as a power plant. and the change AFTER xande was only to produce MORE energy.

Thank you Arixtotle