It would be nice to have proof, but I’m 100% certain the reason was “to see what happens”.
But by the time of CIA's LSD brothel, the CIA had already been experimenting with LSD for over ten years. Timothy Leary and the Acid hippies had already been all over the counter culture for 5 years. Everybody knew what LSD did by then. They already knew what would happen. That's why I believe the brothel wasn't an experiment, it was an operation with a defined objective.
The appealing part of this story was the bit where Sandoz called them back and asked "did you want enough of this to light up the country?" and some ignorant bureaucrat said "that's whats on the form, ship it already." That fits my cynicism about government operations, especially unsupervised ones.
The stories about the government involvement in cocaine are less funny.
I wonder to what extent the CIA experimented with extreme doses if they sat on that much.
I'd guess that a couple of sessions in everybody running the experiment saw the potential to extort, manipulate and just plain have fun drinking/drugging and doing plain old illegal things under the pretense of "experiment".