Not too long ago, just towards the end of last year actually, Google changed up their Play Store policies for app and game developers stating that they could no longer include "user testimonials" as part of the app or game description. In Google's explanation they were banning this because a lot of the times the testimonials could be used to manipulate the way apps show up in search results, by injecting certain words referred to by Google as popular search terms into the testimonials that had virtually nothing to do with the app or game, thus making it easier for people to find those apps in search to download and install even if they weren't looking for them.
This is a legitimate reason to outlaw testimonials as the description should be kept for explaining to users what your app or game is about and further, give reasons as to why any individual would want to download said app or game. While this may charge a more difficult task to app developers and publishers of gathering testimonials through actual app review comments and downloads without "keyword spamming," it isn't impossible to do without the process of testimonials in the app description. Google is essentially trying to make things more fair for app developers across the board.
Without such a tactic as testimonials on how good or awesome an app might be towards the top of the page, what can developers do to gain recognition from would be users and potentially increase their downloads? As already stated one big piece of the puzzle is the actual reviews section. Paying attention to the reviews and feedback left by users who are actually out there using the app is a great way to foster organic app growth. Word of mouth referrals from user to user may not seem like something developers have much control over, and for the most part maybe they don't, at least not directly, but if your app gets any sort of notice from users, and they like it, they will refer it to their friends and it's going to pick up traction socially. When that happens, users will leave reviews and feedback which gives developers an opportunity to naturally respond to what users do and don't like, which ultimately seems like a better solution to app growth than testimonials which Google nor any user has the possibility to verify.
This doesn't mean that testimonials aren't something that should be included somewhere as they can still be an important piece in fostering app growth. Not to mention that developers still have the ability to include testimonials within their app descriptions, only now, in doing so approval of apps being published may get held back since Google is basically asking developers not to use them.