ruby - rails render format- why two formats -

this code generated scaffolding

  def index     @swimming_classschedules = swimming::classschedule.all      respond_to |format|       format.html # index.html.erb       format.json { render json: @swimming_classschedules }     end   end 

why generated 2 formats . seems use html format. when json format used?

sometimes need data in format of json. ruby on rails 1 of must used framework backend of mobile application (android, ios). android , ios support data in format of json. data of json format generated when scaffloding. in case of web application not necessary generate data in format of json.