Cooking with children from a young age can help them learn basic counting and reading skills, grasp the concept of following directions, develop their fine motor skills, and, the thing most parents are focused on around food at that age, encourage an adventurous palate.
Buuuutt... I KNOW, it's also a lot of work to cook with a preschooler. Trust me, I get it. I know the idea of allowing them "help" can seem like it will take too much time. I've discovered, however, that if you plan ahead a bit and go into it with the right attitude, it can be a wonderful experience for both of you.