When, or why, would you eat such a thing?

You might want one to improve your sports performance, to give you instant energy, as a meal replacement, as a snack, or for a myriad of health-related reasons, including weight loss and disease prevention. You probably don't want one because it tastes good. (They are improving.) Then, assuming you do want one, which one do you want?

Energy, from the point of view of the organism (you) is delivered by calories. Those calories are composed of the macronutrients protein, carbohydrate, and fat. Energy bars also contain micronutrients: vitamins and minerals. You should be getting these important nutrients from fresh food, other fortified foods like cereals, and, if necessary, from supplements. The micronutrients in energy bars may be helpful if you are deficient, but you should not depend on bars for your vitamins. These ingredients will not give you instant energy. Energy still comes from calories. Some of the more expensive bars contain such ingredients as creatine, glycerine, amino acids, and mysterious multisyllabic chemicals. These add mostly to the cost in this type of product.

So choose on the basis of amount and type of macronutrient. Although you can get energy from any calories, your body prefers carbohydrates. There are other jobs for protein, such as tissue building and repair, and fat takes longer to kick in. You are generally using a mixture of carbs and fat. You burn a higher percentage of fat at rest, but the total number of calories is small.

Endurance athletes especially need a lot of carbohydrates. When a marathoner "hits the wall" or a cyclist "bonks" they are usually running out of carbohydrate. Although they have a lot of stored fat available, fat requires carbs to be fully metabolized. The body may have to get carbs by converting protein from muscle tissue, and you don't want that. If you're an endurance athlete having an energy bar before, during, or after a workout, make it one with at least 70% of its calories from carbs. Weight trainers may also want high carb snacks for energy before a workout, but they can eat a high protein bar afterwards.

If you want a bar as a snack or meal replacement, look at the calories. Under 200 calories may be ok as a snack, but will probably leave you hungry if that's your lunch. On the other hand, don't snack on energy bars like you would on rice cakes. The average bar has about 250 calories, just a little less than a Snickers, so if you eat four a day, you're getting an extra 1000 calories, and you will have to work out a lot to burn that off.

Energy bars don't replace real food.

Mother Nature still knows best. The bars are a convenience food, although more healthful than most. It's good to keep one with you in case you don't have time for a real lunch, for instance. And they can certainly help fuel your workout.

Can you use high carb food for a pre-workout meal? Sure. And you should do that when you can. When you have to squeeze in that meal, though, the convenience of an energy bar comes through. You can carry it almost indefinitely in your pocket, gym bag, or briefcase until you need it. And you can't say that for a banana.