One of the main reasons I installed Amazon appstore on my various Nexus tablets and phones is for the daily free apps. I have over 500 + so far and some of them are quite good, including those I already bought from Google playstore: swype and switfkey, Runtastic pro, SketchBook pro, etc., and tons of games. And don't forget about the Amazon coins promotions. I have over 500 coins so getting Plex for 99 coins is a no brainer because its free!
Also I remember, a while back, having Android devices that don't include Google playstore nor is it supported. It's much easier to side-load Amazon appstore in such cases. Even with all that I still think Google Playstore is more refined and works great on Google "approved" devices. Amazon has been making some major improvements to the UI lately though.