APKMirror is a website that hosts various kinds of apps that can be found in Google Play Store and can be downloaded without the need of an account.
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Pro Ability to get old version of an app
If the current app doesn't work correctly, users can seek an older version of the app by finding it and downloading through APKMirror.
Pro No account needed
There is no account creation required, so anyone can freely download the app directly from the website.
Apkmirror is run by the people behind Android Police, one of the biggest android news sites. All apps are manually vetted and aproved.
Pro Many popular apps available
There are many popular apps on APKMirror that are also available on the Google Play Store which are contributed by the community. There is a strict check of the app when any user or developer wants to upload the app to the website to ensure that the app is same as on the Google Play Store.
Pro Get access to early alpha / beta version of apps
The website also hosts many early alpha / beta versions of apps, so that anyone can access the beta app without needing to wait to become a beta tester.
Pro Modern war source for all your favorite developers
Modern war source is an open-source code for online gaming and program developing, which can help to minimize the effort of making an online compatible game.
Con No native Android store app
APKMirror only comes as a website and does not have their own Android store app, thus users will need to download the app through the website and then sideload their app manually (requires activation of sideloading on ones device, this can be done by going to Settings > Security > Allow Unknown Sources).
Con There is no compiled 'Changelog history' for apps
There shall be some changelog history, which contains the (maybe official) changelog of the app from earliest to the latest version available on that site (i.e. ApkMirror).