Crunches don't work as many muscle groups as planks but they hit the external rectus abdominus muscles and the external obliques hard, the ones that give your abs definition when they're developed. (cathe.com)
The problem with crunches, apart from big questions about their role in vertebral degeneration, is that they focus almost exclusively on rectus abdominus (the exterior abdominal muscle). (womenshealthandfitness.com.au)
The following ab routine is designed to target each critical area of the abs: upper and lower rectus abdominus, inner and outer obliques and transversus abdominis. (blog.fitplanapp.com)