I use regular Dri-Fit whatever socks for shorter rides, but when it's hot, or I have a long ride, I go with my thin PI socks.

They simply keep my feet cooler. If I had never used the PI socks, I probably would think that the Dri-Fits were fine because the difference is not earth shattering, but once you have used both of them, there is no question the PI's keep your feet noticeably cooler.