Crunches are included in these exercises. But some of us want the extra support an ab machine can provide. The Torso Track II, for example, is a sliding machine and could possibly cause problems for people with lower back problems. Try to pick a machine that will be easy on the back. Don't be fooled by flashy gimmicks.
Some machines use isometric contractions.
This is a contraction that tightens or flexes the abdominal muscle. Other machines use eccentric and concentric movement. This means they lengthen and shorten the muscles to strengthen the abs. Decide which is best. One type of abdominal contraction isn't necessarily proven to be better than another, but they will have different results.