I think the reason was the one Buffy herself stated later in "Checkpoint" - power. Power over the Slayer. Whether or not that was a sane reason is another matter.putting the one person who could save the world in jeopardy for no sane reason is stupid.
I think the Council thought, big whoop, if she dies, another will be chosen. But that's shallow thinking, because a Slayer needs training - some were contacted by Watchers beforehand and trained, but you couldn't count on one of them being chosen. Kendra was trained all her life, yet Buffy was chosen first.she would have trusted Giles and men more. she would not have used her 'thinking muscle' the way she did that night. I think it is a horrible test and shouldn't exist, but I think the idea of mentally testing the slayer is sound. execution sucked. putting the one person who could save the world in jeopardy for no sane reason is stupid. What if Buffy died and they then had to fight apocalypse season without a trained slayer?
the difference between the Masai test and the Cruciamentum is that one strips her of her power secretly and has her most trusted aide lie to her and abuse her. I imagine all Masai young men prepare for the test all their lives and spend time strategizing how they will do it -- that is NOT the same as having your powers taken from you and then having the worst of the worst set on you without preparation or your usual abilities. That's not a way to prepare her to succeed.The Maasai Tribe of Kenya has a custom where a young man goes out into the wilderness to kill a lion by himself. Only after he succeeds, is he considered a man, that is, a warrior, and is worthy of joining the hunt. So, no, the idea of Buffy having some extreme ritual test to complete to demonstrate herself worthy of the powers she is set to fully inherit, doesn't offend me, at least in theory. The ancient world was harsh and cruel, the slayers and the watchers would have to be so as well. Nor do I mind that the Watcher's Council did this to her, after all, what good is instruction if it can't be evaluated? I mean, I wish Buffy would have tossed some of the Potentials into one, would have beaten some sense into them.
The issue is, short term, it did cause Buffy to distrust Giles, long term, I think it did help, as getting fired for a girl and saving her at the last minute did show he was on her side, in a way being a stuffy librarian never would have.
The bigger problem is Buffy and the council, but we never really saw enough of the council to really care that much, other than as a strawman.