The reason is simple. Not all Android devices came pre-installed with Google Play. So if you want your Android apps to reach a bigger audience market you need to install on other third party app stores too. This group of users is too big to ignore. I know cuz I compare the download statistics.
On another aspect, don't you want your app to better other people's daily lives? Do you want to deprive them juz becuz they has no Google Play to install your app? An app is created to be used and it should not be restricted to a selected group of users. It should be for everyone.