Using beans is a very common practice in JSP application development. This approach is popular because JavaBeans introduces reusability. This is to say, rather than building your own piece of code, you can simply use what other people have written. For example, you can purchase a bean for file upload and start uploading files within 30 seconds.
Even more than reusability, JavaBeans offer predictability. Because they follow a standard convention, they can be manipulated by reflection so that things like the standard JSP actions and the Expression Language can act upon them.