The single-arm dumbbell crunch is a more difficult version of a highly popular abdominal movement. Adding a weight to one side makes the crunch far more difficult to your rectus abdominis or "six-pack" muscles, as well as the obliques and deep core muscles.
Allows you to train in lower rep ranges when bodyweight crunches become too easy
Weighted ab training can help the core muscles grow and look more defined
Also challenges the shoulders, serratus anterior, oblique, and transverse abdominis muscles