"Regular crunches" refers to a type of exercise where you lie on your back, bend your knees, and lift your upper body towards your knees using your abdominal muscles. Full definition
The only problem with doing regular crunches off the floor is there is not enough range of motion for you to get a better ab workout so in order to increase that range of motion... (nowloss.com)
When you lie on an incline, you have to curl up against gravity, and using this exercise is a simple way to progress your ab workout when regular crunches get too easy. (ab-core-and-stomach-exercises.com)
This motion will especially engage your lower abs, which is something that does not always happen with regular crunches. (dbstrength.com)