Training your lower back can improve your posture, facilitate the development of your abdominal muscles, and help prevent lower back pain and injury. It can help round out your glutes as well if combined consistently with leg training. Most of us can appreciate that little pick me up.
A great twist on an old favorite is the low-cable back extension. This is a little more challenging than the traditional exercise, but it's a welcome change of pace.
First, position a roman chair or back extension machine inside the cable station. The machine should be squarely facing the weight stack with enough distance between the machine and the weight stack that while at the bottom of the exercise there is still tension on the cable. Attach and grasp a D-handle to the low cable pulley. Make sure the weight is light but challenging. Mount the machine by placing your feet in the foot plates so that your thighs are against the thigh pads and your hip bones are just above the pads. (If your lower back and/or abs are weak, position your hip bones just against the top of the pads and do not use any weight.) With straight arms, slowly lower to the starting position by flexing your lumbar spine maintaining the natural curve in your back.
Keeping your arms straight, raise your upper body while squeezing your glutes until your form a straight line with your body. Your chest should be out and your back should slightly arch at the very top of the exercise. Slowly lower to the starting position and continue for about 15 to 20 repetitions.