In the first episode of "Mighty Morphin Power Rangers," titled "Day of the Dumpster," the transfer of the Power Coins among the Rangers serves to introduce the characters and their roles within the team.
Billy Cranston, originally the Blue Ranger, gives the Black Ranger Power Coin to Zack Taylor because Zack is taking on the role of the Black Ranger after the Rangers initially receive their powers. The episode emphasizes that the Rangers' powers are adaptable, and it helps establish the dynamics of the team.
Zack then gives Billy the Blue Ranger Power Coin to swap roles because of the need to utilize the team's strengths for the battle against Rita Repulsa. This also serves to highlight the teamwork and camaraderie among the Rangers as they adapt to their new responsibilities while fighting against evil.
Ultimately, these exchanges help to set the stage for the series, showcasing themes of collaboration, friendship, and the willingness to support one another in the face of challenges.