Metric junkie here.
Mileage. Avg. speed. Elevation gain.
...and folks here introduced me to BikeJournal.com, so I can feed my obsession on a daily basis.
And I have no clue why, really. It's not like my riding is anything to write home about!
But, yeah. It motivates me. Gives me a goal. Allows me to compare one (ride, bike, year, month, BikeJournal flunkie, etc.) against another (again...for what? Don't know).
And one January 1st, with much fanfare, I will have the "Annual Resetting of the Bike Computers Back to Zero!". Yee ha!