Why would graffiti artist banksy use stencils to transfer his artworks onto walls?

Because stencils allowed him to apply the art quickly as there is risk of being arrested for defacing the public property.

Explanation:

Banksy is a name to an anonymous street painter in Great Britain. He is most famously known as a graffiti master, painter, film-maker, and activist. Banksy's name is still unknown even after being a graffiti master for over 20 years.

His works are inspired by the works of Blek Le Rat. He started using stencils when he was 18 years of age. He made the use of stencils in is art when he nearly got caught by the police for defacing the public property. Using stencils in art would shorten the time to complete an artwork and also reduce the risk of being caught.