Because it might foreshadow some of the later plot. It seems like a mighty big coincidence that Zabuza was leaving the Hidden Village of Mist just as Orochimaru shows up to watch the Kayuga clan attack it. Especially considering that Zabuza was leaving because of his failed coup d'etat against Mizukage. Kishimoto has a track record of turning coincidences into plot points.

Ignore this post. Sorry. I didn't think I hit reply.