From where I am, there are abstractions coded for APIs in the layers of - topmost API layer, then Business logic and the 3rd DAO layer. Even though there is on

From where I am, there are abstractions coded for APIs in the layers of - topmost API layer, then Business logic and the 3rd DAO layer. Even though there is only one implementation everytime of these layers, this structuring alone has helped maintaining the code so much easier, as everyone even across teams goes by this structure while defining any API. Can't even imagine just coding functions in large codebases without a pre-defined structure, it can become brittle over time.