For most of the novel, Nick has a completely erroneous conception of Jay Gatsby. Gatsby is selfish and a criminal. But Nick doesn't notice any of these imperfections in his character. Why not? I'd say because Nick is in love with Gatsby.
Fitzgerald filters the reader's view of Gatsby through Nick's skewed perspective. This is important, because it lets the reader be blindsided by the revelations about Gatsby's true character towards the end of the novel. If Nick wasn't in love with Gatsby, it would be much harder to justify why Nick had such a skewed view of him.