1. Often they have almost the same amount of calories as the originals. (Why? See 2.)
2. When they take out the fat, especially in deserts, they need something else to make it tasty: Sugar. So as the fat goes down, the carbs and sugar go up. You eat more sugar, you get more hungry. Whereas if you ate the fat, you’d stay full longer.
3. You won’t fall into the “but it’s reduced fat so I can have a little extra” trap.
4. You won’t feel stigmatized or deprived by having a cabinet full of products that say “fat” on them.
5. The regular versions often cost less than their reduced-fat twins, so you can save the money and spend it on whatever your heart desires.

This list does not apply to dairy products, which are full of saturated fat and probably (though arguably) better had in their lighter forms.