Because it makes millions of things easier for you. It encapsulates all the underlying web relevant boilerplate (mapping request paramerters into beans, internationalization etc. etc.).
For bigger project a framework such as Struts or other is almost indespensable.
JDBCSupport - An easy to use, light-weight JDBC framework -
First of all you have to understand design patterns. Design patterns is repeatable solution for commonly occurred software problems. Struts is framework and Struts basically developed using two design patterns.
1 ) MVC - Model view Controller.
2 ) Front controller.
so using framework you can make your application development fast and just stay far from some problems at the time of development.