The abdominal muscles are key components to providing strength and support to keep the body upright and for movement.
When these muscles are in poor condition, additional stress is applied to the spine as it supports the body and back injury or pain is more likely.

A strong abdominal musculature may be an important factor to enhance athletic performance and prevent back injury.

Simply exercising the abdominal muscles should be included as part of a daily routine of good health.

Many people perform an infinite number of repetitions when performing abdominal exercises. They are still training muscles and the abdominal muscles should not be treated any differently during strength training than any other muscle group.
People do hundreds sit ups or crunches a day. Would they do hundreds bench presses a day to increase their upper body strength? Of course not, and there is no reason to do hundreds abdominal curls either.
If you are performing high repetition abdominal exercises (sit ups, crunches) you will improve local muscle endurance and only slightly increase strength during the first few weeks of the training program. After that your ability to perform more repetitions will increase but not your abdominal strength.

To strength train abdominal muscles it would be more effective to reduce the number of repetitions and increase the resistance.

TENDO AbEx – F313 offers effective way to increase strength of the abdominal muscles

The abdominal muscles – what we would need to know