Brand New to Xd - Have Only Downloaded It - Need to Understand its Benefits

I have downloaded Xd and will be embarking on my first design in it soon. Looks like this software has been out for a few years now, but I am only just now learing about it and looking to expand my skillset. I have found tutorials, but not something that really explains to me the benefits of this app. I have been designing websites in Adobe InDesign for years. Sometimes in Photoshop or Illustrator if asked to do so. I use Dreameaver for coding, or for customizing WordPress themes. I design and also program in HTML, PHP, and CSS. And do animations via Adobe Animate for HTML5.

With InDesign, there are pages, but one page is typically one design.

I currently have an approved homepage design I did in InDesign. I am going attempt to design an interior page based on the InDesign look for an interior page of the site using Xd. Figured it would be a good way to get my feet wet with this app.

With Xd there are artboards of different sizes. Trying to wrap my head around this. I typically do not share more than a desktop view with the client, and then do the responsive layout when programming. My sites are fairly small and this has worked for me and my clients thus far. I understand the benefits of sharing a phone and table view with a client for bigger projects. I just haven't needed to do so yet.

So I am to create an artboard for desktop and maybe next to it one for tablet and another for phone? Does this software somehow convert a desktop design to an iphone artboard (stacking item that were horizontal for example or changing font sizes).

I've seen videos showing rollover states and animations. Does this export into HTML and CSS? Is the designed exportable to PDF? Or is there an interface where a client can see the design via the web? So they can rollover the buttons? 

Just trying to understand how I am to use this since I am coming from InDesign.

The https://letsxd.com/getting-started website seems to skip over the very basics of this.