There are lots of application that require certain things when the user uses a JavaScript like:

Data entry validation : This tell that if the field of the form is filled out then during the processing of the server the client side can interact with it.

Serverless CGIs : This describes the processes that are not used with JavaScript but programmed as CGI on server, it gives low performace due to more interaction between the applicatioin and the user.

Dynamic HTML interactivity : It allows dynamic position of the data without using any other scripting language.

CGI prototyping : It allows more reduction of time to access the user interface before implementing the application of the CGI.