I’ll give you the hook first, then the explanation of why it’s important: From everything I’ve been able to gather so far, AEM Communities is not being ported to the AEM Cloud Service, and at this point looks like a dead product which will not be getting further updates.
Now here’s the back story, and why it matters. Remember the ye-olde term “Web 2.0“? Somebody said it in 2004 at an O’Reilly conference, and then name stuck: it implied the ability to use the web not only to consume content, but to interact with it in terms of social media, comments, ratings, uploads and other user-generated content. Fast forward to today, and social media is an entirely inextricable part of the Internet experience, even when it comes to traditionally non-interactive and “not-very-web-2.0” properties like marketing sites and corporate about-us pages. Truly-immersive experiences many times need to accommodate not just pushing content out, but allowing people to interact.
For experiences built off of Adobe Experience Manager, unless one wants to create a UGC framework and infrastructure system from the ground-up, the solution to handling this is AEM Communities.