I don't think anyone reads the manual for anything - I like to, but I'm probably in the small group.
That being said I'm sure that there are some people, that even if they read the manual, couldn't understand / make anything of it and those are the people the Genius crowd help. Some people also prefer customer service / human support as well.
I think it's ironic that BMW, a brand supposedly all about the driving experience and performance, needs a Genius in the dealership. The whole brand is kind of built on 'those that know' or the enthusiasts like you and I who read manuals and yet I'm willing to bet Kevin has a job because the brand is so damn diluted and distracted that 80% of customers are of the clueless, confused, badge-and-status-obsessed crowd