A friend's hands can be very useful things to have "handy" when you're out photographing--they can help you carry gear, point to interesting subjects that you might have overlooked and, if you're lucky, maybe even scratch your back occasionally. But having an extra hand or two nearby can also be a useful creative addition to your compositions. Hands can be used to show comparisons of scale and texture, they can hold unusual objects in your compositions for you (a plastic alligator dangling over Times Square, perhaps) or maybe just to point to a real alligator sunning in nearby in a stream (hopefully not too nearby). Hands are interesting because they add a human element to any type of composition and again, in terms of both scale and texture, they can be extremely helpful. Using my friend's hand touching this giant saguaro, for example, provides a great sense of scale to the massive folds of the ancient cactus (probably 100 years old or more) and her thin delicate fingers provide a wonderful textural comparison to the rough, time-worn skin of the plant.
By the way, if you aren't lucky enough to have a cooperative friend with nice hands nearby, you can always use your own hands--just place your camera on a tripod, frame the scene and then use the self timer to get your hand(s) into position. Or, if you have long enough arms and a wide-angle lens, you can probably even just reach out into the scene and include your hand. I've photographed small stones and shells sitting in the palm of my hand this way and have even photographed a chipmunk feeding from my hand. But however you get a hand in the shot (and whosever hand it is), just be sure to frame the scene carefully so that people know you meant to include it and didn't just take a nap while you were composing the picture. You've got to hand it to hands--creatively and logistically, they're quite handy!