Your core muscles are crucial to holding your whole body together. They enable you to stand up straight, to squat down, to bend over, to reach up, to twist and turn, and to avoid lower back pain. Healthy core muscles (and this includes the muscles in your lower back as well as your tummy) are super important for a healthy and strong body.
So doing crunches will do virtually nothing for burning fat, but they will help your body to become better and healthier at what it does. (But avoid the ab machines. Do planks instead.)
"Losing weight is easy; I've done it dozens of times."
(with apologies to Mark Twain).
Ultimate goal: Size 10 clothing.