Sideloading APK files: Pros and Cons

For majority of Android users, Google Play store is usually the go-to marketplace in which you can officially download tons of applications without fear of malware and viruses. However, the download on Google Play isn't always pleasant since you may encounter several unwanted errors while attempting to download apps, thereby finding an alternative source will be an optional solution in that case. When it comes to APK files, lots of people tend to say "No" or refuse to download and install it, stating the reasons that those files are extremely risky and dangerous to their devices. But if you get yourself a reliable source to download original APK files, the third-party installation is no longer a concern.
The risks of sideloading APK files
When installing apps from Google Play, you're fairly secure b/c the apps are verified by Google before officially publishing; however, no one can absolutely guarantee the safety of an APK file from other unknown sources. Your device is likely to be at risk if you just randomly sideload a suspicious file which is intentionally tampered with. Thus, having at least one anti-virus software to detect virus and spyware is highly recommended.(VirusTotal)
The benefits of sideloading APK files
Sideloading APK files will be advantageous in some cases below:
Google Play is not responding.
Download different versions of an app (beta version, older version or latest one).
Download geo-restricted apps such as Hulu, Melon, Pandora, etc.
Download apps that are no longer available on Google Play due to inappropriate content (deleted apps) such as Flappy bird.
To experience Android apps on different platforms: PC, TV, Android wear and Android car.