A good hand lens is a fundamental part of a geoscientist’s toolkit. It is small enough to be carried around anywhere, and makes it easy to see details of the rock.
What aspects of the rock are important that might be missed without a hand lens?
Sometimes the minerals (grains of a certain chemical composition) are too small to see clearly with the naked eye. With a hand lens, these minerals can be identified, leading to clues about the chemical composition—and thus potential origins—of the rock. In the case of the photo above, the identification of vivianite gives clues about what conditions were present in the lake when it was deposited (lots of iron and phosphorous available; what scenarios would produce these kinds of conditions?).
Another thing to look for, in sedimentary rocks, is the degree of rounding of the grains. Angular grains have not been smoothed much by wind or water, so they would have come from nearby. More rounded grains would have had more tumbling and pounding from transport by wind and water, so are likely of more distant origin. Sediments can also contain small fossils, which can be seen with a good hand lens. Identification of these fossils may enable a rough estimate of the age of the sediment.
For metamorphic rocks, a hand lens may reveal foliation, reaction rims, and even compositional banding in gneisses.
I use a 10x hand lens, and it works well for me. It’s small, light, and I have it on a nice lanyard.