Why is the tension between Osiris (and then Horus) and Set important in the Egyptian Creation Myth?
Ra creates the world and the other gods. Osiris becomes the ruler of the world.
Osiris marries his sister Isis (a common practice among Egyptians to preserve the royal line). They are very happy; Set (or Seth—their brother) becomes jealous and angry, so he murders Osiris. First he places him in a coffin and puts the coffin in the water, but it flows to such a place that Isis can find her husband's body. Then Set cuts the body up into many pieces and spreads them over the land of Egypt. However, Nephthys (always loyal to Isis—and her sister) joins with Isis to find all the piece of Osiris' body. When this is accomplished, Isis puts the pieces together and uses her magic to breath new life back into Osiris. He goes to the underworld and decides to stay there.
In the meantime, Set decides he will rule the land, but Osiris and Isis' son, Horus, grows up to challenge this. He feels that he should be ruler, not Set—the murderer of Horus' father, Osiris. The two gods participate in a number of challenges, but Set cheats. During the last "tie-breaker," Horus cheats, but the other gods declare it is a tie. So they contact Osiris in the underworld to ask him. He insists his son should be ruler. Set used murder to achieve his place on the throne, but Horus has never done so. So Horus becomes "the ruler of the living."
The Pharaoh of Egypt was considered as the living Horus.
The conflicts between Osiris, Isis and Horus with Set describes how these gods came into power; they are a part of the creation myth in order to explain how Horus becomes the ruler of living men (and the god-like essence residing in all pharaohs), and how Osiris becomes the god of the underworld.