As a CO elite, you are entitled to premium seating in coach when flying CO. The only thing premium about the seats is their location close to the front of the plane, but it's still a nice perk. Also sometimes when there are only middle seats left in back, you can secure an aisle or window seat in this section.

If you were flying on a NW flight I'm surprised that you got moved up. In general, CO elites do not get access to NW premium coach seating, though I have personally selected NW premium coach seating on some NW flights at time of booking, though I suspect this was due to NW unblocking those seats closer to flight time (these tickets were purchased 2 days before the flight). Your mileage may vary on NW.