I guess we need to learn how to look at a story line, gravemind plays a considerable role in the halo universe, hes the catalyst for many of the events that happen in the game and is a key player in the story line.
Link theres the halo wiki definition, it should fill you in on anything and everything he's ever done.
That is indeed very interesting...
It seems to me he played a larger role in Halo 2 than Halo 3 - in H3 he's almost there to simply annoy you. The only significant part he plays in the story is when the Flood join you in helping kill Truth, and then he turns on you.