Even if your focus is on that chiseled 6 pack, it is a good idea to build and maintain strength throughout the entire core belt and adding the superman (any injuries preventing this aside) to your routine will help your lower back development It's important to keep your core strength balanced so focusing only on sit-ups and crunches is a huge mistake since they primarily work the rectus abdominus. Aim to also extend the lower back (eg. bird-dogs and superman) and add twists