There might be other reasons, but the ones I have mentioned are probably the major ones. Why applets are important I can understand why Sun initially believed that client-side Java was important. It tried to introduce a new paradigm for the development and delivery of applications, which I believe was the company's attempt to reduce Microsoft Windows's popularity on the desktop. The applet model is actually a fantastic one. If Sun had executed on that vision successfully and Netscape had supported that effort as it had publicly promised, applets would have been a smashing success by now.
Applets are still very much needed.
In fact, the notion of a page for each key function in a Web application can get downright annoying for some applications, particularly if you have a slow connection or network traffic is high. Applets are a great solution for applications that have several screens and many interdependent input fields. In addition, applets provide direct access to Java technologies such as JDBC, RMI, and EJB, which can result in more efficient computing. It is one of the best examples of how the applet model can serve as an alternative to traditional Windows or even Java standalone applications. The use of applets such as infoBook in upcoming Internet appliances and special devices such as Web phones and automobile navigation systems will become very important.