One reason to use a third-party app store on Android is that certain apps do not comply with Google Play's terms and requirements, but that does not mean that they are risky or dangerous. Another common reason is that the developer saves money using third-party app stores. After all, Google obtains a big chunk of commissions and other expenses, partially because of its dominance in the operating system. Regardless, there is still a risk whenever you install an app outside of Google Play. Downloading an app from places other than Google Play is referenced as sideloading. Recently, interest in sideloading rose again with the global popularity of the game Fortnite. The game is available for Android, but you need to download it from the official website instead of the Play Store. Of course, this decision was by choice due to Google's policies and costs.