Apart from why "those apps are not in a store", there's very good reason to want to use an alternative source for your applications. F-droid is a far safer source than google play is, because they actually vet the source code and project and build it themselves. You are far more likely to download malware from google's official 'safe' sources than from F-droid, and hence it's my first option when searching for simple utility applications because the top results on google play will be utterly infested with ads and tracking at minimum.