An example of a web application is an invoicing system, billing system, or customer relationship management system.
The beauty of web applications is that they can often get your client to perform tasks that otherwise you would have to perform on thier behalf. This saves you time and money.
If your clients could connect to your website and view the status of thier order, make changes, pay thier bill and view thier last invoices, imagine the reduction in phone time for you and your staff?