I carry more water than I need on desert trips. I've almost never needed the extra that I've brought, but I'd hate to have a time where I ended up needing it and decided not to bring it.
Otherwise, it's just clothes. I always like to bring some changes of underwear, socks, and shirts, even for a night or two. I bring deodorant to keep the stink down, but sometimes a shirt change is just needed.
We brought almost 9 liters each for 3 days in Needles last October and ran out by the time we got to our last site, SQ1. We were parked at Squaw Flat and had a cooler full of Gatorade. Another guy and I hiked out to Squaw flat, loaded up on water and raided the cooler and hiked back to our site. I still chuckle about that.