From the Super Mario Wiki
The Power Platform is an arena Bowser uses during the final fight in Paper Mario, powered by Kammy Koopa's magic. It is modeled after the Koopa Clown Car. Using this Platform, Bowser is able to increase his size and power, becoming completely invincible by preventing the Star Spirits' Star Beam from taking away the power of the Star Rod. However, Princess Peach can combine her power with the Star Spirits to create Peach Beam, which can nullify Bowser's invincibility once again. After Mario Co. defeat Bowser and recover the Star Rod, the Platform begins to malfunction, destroying it and Bowser's Castle in the process.