This filter works similar to the sharpen filter and therefore is used in the same fields. However, Unsharp Masking is much more sophisticated, as it works by first creating a special mask layer to limit the sharpening only to high contrast areas. Its results are of a higher quality but the speed is slower, too.
Small filter radii - often in combination with a high intensity - are used to increase the sharpness in an image.
Big filter radii in combination with a small intensity can be used to increase the local contrast in an image. The result is similar to "clarity" known from other software.