While we focused on movements the ab muscles make for us, they also support the contents of the abdomen, support the spine, and help with breathing. It's also important to remember that the abdominal muscles work together. If one is to achieve a high level of functional fitness, working out with a balanced approach to toning the abs will be much more effective than over-working any one area, the rectus abdominis, for example.
The abdominal muscles are core muscles, but they are not the only muscles we refer to when we speak of core training or core strength. The core group also includes:
Anatomy of Movement, Calais-Germain.