The step up is one of the best exercises to strengthen your gluteus medius and tone your leg muscles. Weakness in the gluteus medius can result in knee and back injuries. Weakness in the gluteus medius also contributes to instability when you walk, run and jump.
Step ups will also improve your dynamic balance. Its not that easy to balance all your weight on one leg (with good form) while stepping up onto a platform.
1. Face a bench or platform (knee high) and put your right foot on the bench. Your knee should be bent 90 degrees. Don't do this exercise with dumbbells until you have mastered the bodyweight version.
2. As you lift your body on your right leg, keep your body erect (don't bend over) and look straight ahead. Balance on your leg for 1-2 seconds and descend. Don't let your left foot touch the platform.
3. Do all required repetitions on the right leg and switch to left leg.
You may have to start with a platform lower than knee-high. Your hamstrings and butt will be worked more with a higher platform.
Exercises like the step up are important because it uses the same pathways that it takes to walk or run. Plus, one leg cannot compensate for weakness in the other leg. If your left leg is weaker, it will be forced to "catch up" to the right leg. Muscular balance is important to avoid injuries.