Advantages: You can download any version of the app directly from third-party websites. They may have app archives of most versions and you can download the one you may need. Downloading is instant unlike the play store, no need to wait for the verification process, etc. You will have an APK file in your memory card/system memory once you download it. So you can uninstall and re-install as many times without needing to download. Disadvantages: Downloading apps from third-party sources usually is not verified by Google. So can be harmful to your phone. APK files can have viruses that will steal data from your phone or corrupt your phone. Your apps will not be updated automatically as the Google play store usually doesn't have access to it.