It's easy to see how inline skating can strengthen your lower body: you work your quadriceps (muscles in the front of your thighs), hamstrings (back of thigh), gluteals (buttocks) and lower back muscles.
But you can reap some benefit above the waist as well. Simply by pumping your arms vigorously in rhythm as you skate, you'll strengthen your upper arms and shoulders.
Inline skating has even more going for it. It can be excellent cardiovascular exercise. And it's much easier on the joints than running is.