selectedApplication is used to store what application the user wants to search ie, communities, files etc. the application parameter is used by the developer to predefine what application is to be searched so that , for example, communities would be searched by default. since the application is predefined, there is no need for to display the "select application"  ui of the search box, so the application parameter is also used to change the template, if I just used this.application i would not be able to tell which template should be used.