Do you mean what did he contribute IN the original game, or do you mean how did his Crisis Core character contribute to the events in the original?

Because in the original he was a very minor character, and mostly served as being the main "twist" -- Cloud believing he was actually Zack.

As for Crisis Core, that game was really its own story. The only connections the developers made with Final Fantasy VII were forced. Otherwise, it really was mainly about Zack and his relationships with Angeal and Gac-er, Genesis.