I don't know if someone wrote this yet, but remember in NKABOTFD, Giles told Buffy he was destined to be a watcher. Whether that meant as a normal watcher, or as the watcher to the slayer we don't know. But I have always figured that the it was destiny for Buffy to be the slayer and for Giles to be her watcher.
When Buffy got kicked out of school, Joyce made a comment that Sunnydale was the only school that would take her. I think it's because the council used it's power to make that happen. Even if they didn't know Sunnydale was a hellmouth, they probably knew it had crazy things happening there so they wanted Buffy and Giles there. And let's be honest, it wouldn't surprise me if the council knew Sunnydale was the hellmouth, and sent Giles there to be Buffy's watcher without telling him. They aren't generous with information, especially with Giles for whatever reason.