The transformation ability is what makes Ben 10's story unique. It enables him to face a wide range of challenges. Different alien forms mean different strategies. For instance, Diamondhead can create barriers and projectiles, which is useful in both offense and defense. Ghostfreak's intangibility can be used for stealth or surprise attacks. This variety of powers means the story can take on many different directions depending on which form Ben chooses, and it also attracts a wide audience as there's always something new and exciting with each transformation.
Ben's ability to transform is central to the story. It allows for different types of adventures. Say, if he needs to explore underwater, Ripjaws is the go - to form. And when he has to outrun someone or something, XLR8 is there. This constant switching between forms based on the needs of the situation creates a lot of action - packed and exciting moments in the story. Also, it shows his growth as he learns to use each form more effectively over time.