Since I tend to buy at least a week's groceries at once, if there's someone in line right behind me with a very few items (and the cashier has not yet begun scanning mine), I want them to go before I do. Just seems the right thing to do, plus a lot of the time it serves to give me more time to unload my cart to the belt.
Or if I'm in line at some fast food eatery and am having trouble making up my mind, I want the person behind me to go before I do, since why should they be held up by my indecision?
People always seem appreciative of this, and those times the tables are turned, I am grateful for the courtesy as well.