Google Play is overcrowded with apps. It has become almost impossible to make a break for indie developers.
There are many alternative app stores out there and developers shouldn't ignore them. On the other hand, these stores are also a great point for everyone to find apps that aren't on Google Play. More than enough reasons to look closely at these stores. I placed our apps on a lot of other app stores from the very beginning, including ones in China and in Iran. I will talk about my experiences with these stores and how they performed.
If you are Android developers you'll be able to know where else can you publish apps (to earn more) and what to expect there, which stores are definitely must have.
If you are not Android developers but you have Android devices you can find out where you can search for apps that might not be on Google Play.
If you are neither Android developers nor have Android devices ... well, you are beyond my reach. :-)