Taika Waititi was showing off visual effects from this upcoming sequel, Thor: Ragnarok, and one main scene that he screened for us involved Thor meeting Korg on the gladiator planet of Sakaar.
Originally a villain, Korg has history with both Hulk and Thor, having been defeated by the latter in an early story, but then also teaming with Hulk during the World War Hulk storyline when — and stop me if you have heard this before — Korg is banished to Sakaar to fight in gladiator battles.
So yeah, this is why Thor: Ragnarok needs him around.
Korg, who was first introduced in the pages of the Marvel Comics in 1963, is a member of the Kronan race.