Many images can use a little punch to bring out facial details or add contrast to midtones without affecting the highlights and shadows. Before Lightroom, it was necessary to use Photoshop's Curves tool to add detail and contrast to mid-range colors. Using the Unsharp Mask Filter to add sharpness would follow this step.
With Lightroom's Clarity Slider, all that is needed is to move the slider up or down till the proper midtone contrast is achieved. The picture below is a little flat in the midtone colors. The bright sun has washed out some of the richness of the colors and detail in the corn and face could use a little punch.
See how the image below shows how moving the Clarity slider up to 43 has increased the midtone contrast. The face is more defined and edges of the corn stalks are crisper than before. All the detail in the midtone range is sharper and crisper.