As a CO Flyer:
1. Upgrades
Unique to CO/NW:
1a. Free companion upgrades for Gold/Platinum Elites-- impress your friends and make new ones ;-)
1b. Free upgrades into Mexico/Central/South America
2. Mileage bonuses
3. Nowadays, First Class Check in and FC line at Service Centers for flight irregularities
I don't value pre-boarding very highly because I'm very successful at getting upgraded, thus FC will have space for my carry-ons well into the boarding process.