I usually have to take a flight at the end of the year to lock in Diamond on Delta. What does that buy me? Upgrades to first class on just about any domestic f

I usually have to take a flight at the end of the year to lock in Diamond on Delta. What does that buy me? Upgrades to first class on just about any domestic flight I take. The exception is DTW-SFO where first class is full of the ultra-Diamonds who commute to work from Detroit to Bay Area. (not really a higher class, they just book way in advance to get best fares)