Cyclists participate in a sport that tends to be on the endurance side of the track. We need energy and lots of it. Eating a big plate of spaghetti for dinner only goes so far. All the energy that is burned off needs replacing, and eating an energy bar is a good way to do just that. Nutritionally, energy bars are appropriate for exercise.
Many cyclists find the high carb bars easier to digest during rides.
And you're still probably wondering what the big deal is about energy bars. The big advantage to taking along a bar on a long ride is convenience and digestibility. Of course, those two factors really depend on personal taste. What tastes good to one person might make another person want to puke, especially on a really tough ride on a hot, humid day. And convenience is really up to personal preferences.