Cross Domain Call in SharePoint Hosted app.

Hi, I am very new in SharePoint 2013 App dev and want to understand when actually Cross domain calls are required and how we can achieve it. 

Getting host web site title from a sharepoint hosted app needs a cross domain call?

My point of confusion is some places I have seen we have to load SPRequestExecutor for getting data from host web  but I am able to get it using changing the context to host web and then getting the title without using SPRequestExecutor:

 appContextSite = new SP.AppContextSite(ctx, spHostUrl);

   Nweb = appContextSite.get_web();
   //Nweb = ctx.get_web();
   ctx.load(Nweb); 

What is the difference between the two( using SPRequestExecutor and not using) and what places we need to use it and where we can get data without it ?

please help me to resolve this confusion.

Thanks