To make sure they have enough special needs/UM seats. I think it is just a percentage thing. They keep enough seats blocked to try to not have to move pre-assigned seat folks once everyone shows up at the airport.

As a side note, I LOVE row one. I mean LOVE. For any flight other than a redeye I'd take it in a heartbeat. I think I had it half of my AYCJ flights. The TV screens on some planes are smaller but the extra leg room for free is a great value, IMO. Plus you're pretty much first off the plane which doesn't suck. And since they aren't assigned until the airport they are often available.