3. Strengthening the core
Planks are perfect for targeting the entire system of abdominal muscles: the transverse abdominus, rectus abdominus, external obliques, and even the glutes.
You need to squeeze all of these muscles to keep proper alignment throughout the body. As you move through variations, you are moving from the core (which is what we should be doing 9 times out of 10) to turn, twist, bend, and reach.
Moreover, planks will incorporate all of the abdominal muscles, erector spinae, glutes and quads. They are number one for working the entire core, so you can tighten your belly.