Longevity. I build a computer once every 6-10 years and don't do much upgrading in between. Buying a beefier GPU means it'll play new games well for a good long while.
And it's not like I'm doing this all the time. I was curious about the power usage, so I made a script to monitor it and starting tinkering with settings to see what the delta would be. I'm a 100% ultra, max resolution 95% of the time.