Rabbits form a species which is prey for several carnivores. Rabbits do not have any means to fight with their predators. Their only means of protection from them is to detect them fast, and run away from them. The long ears of rabbits help them catch sound waves in a very efficient manner. They are like the ear trumpets which were used in the past to hear sounds in a better manner.
In addition to providing an excellent means to improve their ability to detect predators, the long ears of rabbits also help to keep their body cool. This is important as the rapid speed with which they have to run to escape from predators increases the body temperature very fast and a large surface is required to eliminate the heat.