A seal sets a distinction from all other things. Now a Seal among humans and prophets bring about the exquisite perfection of such a being without whom, the example of leader and follower would be in vain. A true leader is the one who not only guides his people in the right path, but also set examples for them to follow; such examples which are priceless if only these peoples put those actions into motion. The followership emotion derived from such leadership is simply amazing and beyond definition.
If the Quran is a Book sent to mankind; not only Muslims, then Prophet Muhammad (pbuh) is also not only a prophet for the Muslims alone. As a Bringer of good news and a Warner he came like his brethrens from among the prophets of the past, including his ancestor and patriarch Abraham (uhbp) to preach the words of God Almighty so that humankind may reform and connect with their Maker. Prophet Muhammad (pbuh) came as a light to shed to pieces the darkness of unfaith and immorality.