Single-leg strength and stability are important and often lacking physical qualities for many clients. Poor movement patterns and weakness can even result in knee pain for some. Performing a proper step-up can increase single-leg strength and stability. However, just because you can climb stairs, doesn't mean you are capable of performing a step-up properly. Recently, I had to re-teach a runner how to climb stairs and perform a proper step-up, as he was having knee pain while climbing stairs.
The video below demonstrates proper technique. Also, the step-up can be modified (step height, resistance, angle, etc) based on an indivdual's needs, and can even be replaced with other single-leg or split-stance exercises when appropriate.