Styles provide a way to effectively reuse hard-won domain and engineering knowledge. Styles represent application-independent rules and decisions that can be applied to many systems in the same domain or product-family.
Styles are key in preventing architectural erosion and drift. Because style-based design decisions are documented up-front and pervasive through a system, they provide a way for future developers to extend a system without eroding its basic architectural principles.
Styles provide a bridge from quality needs to design. While identifying desirable qualities is easy, figuring out how to "design in" those qualities is often not. Styles provide guidance about how to design systems that meet specific targeted quality goals.