What is the point of a DeFuser?
While not the most sought after powerup, the DeFuser can certainly help you complete certain challenges.
This powerup will replace a critter card (like a wormburner or bomb) with a normal numbered card. Since many challenges require you to play a certain number of cards in a row, having a critter card in the middle of the board can actually be a hindrance toward playing the number of cards you need, to complete the challenge.
In those cases, removing the critter card with a DeFuser is the way to go!