A Question about Layered Prorgamming

Now I worked in a small company, in the company, we use struts and a home-made persistent layer framework, we wrote business logic and sql in action, and call the persistent layer framework which we made. And I know this is wrong, because we must separate business logic from action, but I can't see any advance from which , and if we do so, we must maintain more. Even struts they hate to use ( not include me). Does anyone can explain me why layered program so important?

分层的重要性在于解耦.

通常来说 "数据库访问层" 的数据访问方式是稳定的.它的变化比较缓慢.
而 "业务逻辑层" 的业务变化通常是比较快的.也就是说在一个系统中存在 不变 和 经常变 的模块.
如果"表示层"/ "数据库访问层" /"业务逻辑层"都写在一起,对于项目以后的维护修改将是一个恶梦.
而分层的好处就在于将职责不同的模块进行分离,降低它们之间的耦合度.使得系统更加灵活.