Sustained cycling is intense exercise. Your body burns through its glucose reserves rapidly, and if you run out of energy the effect ("bonking") is awful. On the other hand, trying to eat a bunch of energy and having too much in your stomach will make you sick and is also awful. So you do in fact want to ingest the most you can process (but no more) per hour, particularly for extended events.

Nutrition doesn't matter for rides up to 15 miles, and you can improvise almost anything for rides up to 40 miles. More (or particularly hilly courses, etc.) and it becomes very important to get the amount you eat and drink right.