We want employees to be in the zone as often as possible. Mandating specific times they need to be in the office hurts the chances of that. Forcing me in the office at 9am will never, ever get me in the zone, but half of GitHub may very well work best in the morning.
By allowing for a more flexible work schedule, you create an atmosphere where employees can be excited about their work. Ultimately it should lead to more hours of work, with those hours being even more productive. Working weekends blur into working nights into working weekdays, since none of the work feels like work.http://zachholman.com/posts/how-github-works-hours/