The importance of strengthening each muscle group cannot be underestimated either, for all of these groups serve their own purpose. If you strengthen these muscle groups you will notice:
Transverse abdominis: increased ability to lift heavier weights.
Rectus adbominis: improved sports performance, particularly with jumping. This muscle group is also responsible for giving you the renowned six pack look.
Oblique muscles: improved capacity for stable side-bending and waist-twisting
Glutes: a supported back and a strong, shapely booty.