I read somewhere that Ab exercises do not help you lose weight (fat) in your stomach. If not, then what are those types of exercses for? What are the ab machines for? Why do the fitness instructors say.. "tighten your abs!" during workouts?

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.)

If you never conditioned any of the muscles on your body, you would be a weak, deflated, flaccid skeleton when you got to goal weight. The more you strengthen the different muscle groups, the more efficiently your body will perform, and the less you will jiggle when you move.