Hal Jordan went through a lot in the 90's. DC Comics decided to make him go crazy and destroy the Green Lantern Corps. Then they made him a villain named Parallax. The fans did not respond well to these changes, so DC decided that maybe they had done enough to Hal. Hal got to play hero once more, saving the Sun but dying in the attempt.
With Hal dead, that left him free for some redemption as a character, and the best way to do that? Make him the new human host for the Spectre! As the Spectre, Hal handed out redemption, not vengeance like the previous Spectre, and did it in his nifty Green Lantern-inspired costume. The new direction failed to catch with fans, who preferred Hal to act like Hal, and not a weepy, introspective Spectre. Eventually DC gave up and Geoff Johns restored Hal as a Green Lantern, leaving some other sucker to become the host for the Spectre.