Web application scripting in ASP/ PHP/ JSP/ .net allows the user to perform complex actions through a web interface. Services range from basic DHTML and HTML to the more complex COM components & Java applets. Data services store retrieve and update information at a high level. For web applications, databases are most practical. Databases allow developers to store, retrieve, and update categorical information in a systematic and organized fashion.
Programers develop web applications to make processes easier for you on the back end and your users on the front end. If you have to make frequent changes without having to call on your programer, there are applications that will allow you to login on the back side and make edits to content (Joomla). If you would like your users to purchase securely using a Shopping Cart, there are hundreds of applications for that as well.
Utilizing web applications for your website means greater ease of use not only for you, but your customers. In the long run, web applications show a significant rate of return on the initial investment for installation.