Many Illustration apps include Artboards and Pages today because this is a very important feature (Sketch, Illustrator, …). I don't want to always switch apps for a design process. Especially to create mockups I want to have them all in 1 document and not spread over several files for different programs.
A multi-page PDF will be shown with individual pages in the file system and with QuickLook Plugins on the mac you can even have more options for previews.
I have to be honest, I haven't used AD a lot in the past few weeks because of the missing Art Boards. AD is a great tool but I if it doesn't include Art Boards, I will never switch away from Illustrator and Sketch and will certainly not use AD - which is a shame.