The Vacuum Block is a key item that allows Bowser to use the Vacuum command in battle.
Vacuum Block

When Bowser meets Broque Monsieur, the latter is stranded on a small island off the coast of Plank Beach. Broque Monsieur implores Bowser to rescue him, but Bowser is unmoved. Just as Bowser is about to leave, however, Broque Monsieur offers the Vacuum Block as incentive for his rescue. Seeing this, Bowser agrees to assist Broque Monsieur. Picking up a small rope that is tied to the island, Bowser drags Broque Monsieur to shore with the help of Mario and Luigi.

The Vacuum Block allows Bowser to use the Vacuum Shroom's power offensively. Bowser can use it to uncover the hidden Blitties during battle and to steal various helpful items from monsters. Additionally, it can be used to inhale some entire monsters, forcing the Mario Bros. to fight them in Bowser's Body. This last function features heavily in the final battle with Dark Bowser.