Back pain can often be prevented/improved by having a strong back and abs, true. However a "6-pack" is a function of low body fat for the most part. Crunches(especially unweighted) are a terrible way to make your abs stronger.
Good ways to make your back and abs stronger include squats, deadlifts and progressively loaded ab work.
Good ways to make your back and abs stronger include squats, deadlifts and progressively loaded ab work.