I always keep a few protein bars in my car. (I found a brand that is under 200 calories, low in fat, and actually tastes good.) It's good to have a non-perishable option available for emergencies--like leaving the packed lunch at home or the two o'clock grumblings. Good for post-workout, too.
I actually helped a woman at the gym a couple of weeks ago with my stash. She was on the treadmill--under fueled and under hydrated. She got dizzy and fell off the moving machine. Between my protein bar and the gym's cool water, she was back on her feet (and going home for real fuel) in 20 minutes.