The main advantage of using a layered architecture in software design is that it promotes modularity and separation of concerns. By dividing a system into distinct layers (e.g., presentation, business logic, and data access), developers can work on individual components independently without affecting the entire application. This separation facilitates easier maintenance, updates, and testing. Additionally, layered architecture enhances scalability and reusability, allowing teams to integrate or replace specific layers as needed. Overall, this approach leads to more organized and manageable codebases, ultimately improving software quality and development efficiency.