The CLIENTJSMETHODCALLER is an invisible component that executes JavaScript statements within the client when triggered. When defining the value of WITHCALLBACK with "true" then the result of the method it transferred to the server application.  

It is used for interfacing on client side to other JavaScript programs which may be part of the client environment.

You can add additional own JavaScript libraries into the client processing (e.g. by using system.xml definitions) that provide JavaScript methods that are not bound to the CaptainCasa client processing.