I guess that many of those "productivity freaks" have toddlers, they tend to wake up at 6.30-7. So waking up at 6 is the one hour in your day that is yours and yours alone. So they wake up at 5-6am, to have a couple of "good hours". Because after the morning preparations and a 40min drive to get the kids to school, your mind is less 'fresh' than what it was at 6am.

It's a practical thing (for those who find it to be). Not a panacea for all.