Prophet Muhammad is known as being the prophet that established Islam. As Allah's final prophet, his mission was naturally the greatest. He spread the studies and writings provided to him by Allah, those of which were later compiled into the present-day Quran. Since the Quran is the very foundation of Islam, proudly declaring its most prominent rulings, it is said that Prophet Muhammad was responsible for sharing Islam as well as making it known.
(I know you said to give something other than his last revelation, but it really does explain the reason that he is important. On its own, the fact that he spreads the final revelation isn't what is significant; it is the contents of his last revelation that is key to your answer.)