In iOS5, Apple introduced the UIAppearance protocol to allow developers to customize the appearance of system-provided UI components like UINavigationBar and UITabBar across an entire app or for specific states. The UIAppearance protocol provides a way to set default appearances and styles for these components that will apply universally without needing to change each instance individually. Developers can now customize colors, fonts, and other visual properties for a more unified look using fewer lines of code.