In the field, a rock hammer can be a very versatile and useful tool. One of its primary purposes is to give a sense of scale to photos which otherwise would lack one (see above). A related use is pointing to a specific feature in an outcrop photo, such as an interesting layer of sediment or a fossil.
Finally, and perhaps the use most people would think of, is to make big rocks into smaller rocks (while wearing appropriate eyewear and other protective clothing). Often rocks at the surface have been subject to weathering from sun, wind, and rain. To get to fresh, unweathered rocks, it is necessary to dig back into the rock face. Upon reaching fresh rock, the rock hammer can be used to break off smaller bits that can be analyzed back in the lab.