I thought the Episode was solid. Was just as much of a character development for Charizard as it was for Ash. Both realized that they needed to grow stronger so Ash decided to leave Charizard to train while he worked to become a better trainer as he relied on Charizard far too often to bail him out of a sticky situation or tough Gym Battle(which Misty mentioned near the beginning of that very episode(kind of foreshadowing)).
It was also done so as to give Ash the means of catching Cyndaquil, the Johto Fire starter. Since the Johto games were relatively new back then, they used the opportunity to promote the games by giving Ash the regional starters like they did back in Kanto.