He was the boss of Team Rocket. The entire game, you are going up against all these thugs for the mysterious Team Rocket, with some boss hiding somewhere. Then you find out he's a gym leader!
So you don't just fight him in some confrontation in the top floor of a tower, you fight him in his own gym, and you've been on the path to fighting him all along! He's an opponent you must face on your quest to beat the Pokemon League AND in your quest to beat Team Rocket!
PLUS he's in Viridian City, which is a place you've already been, but overlooked. He was right in front of you all along!