If you own a mainstream android device, you're probably not going to ever have to. If you have something a little weirder, the app store often will not let you

If you own a mainstream android device, you're probably not going to ever have to. If you have something a little weirder, the app store often will not let you install an app which doesn't state compatibility. Sideloading the APK more often than not works fine. Also, there's alternative sources like F-Droid which have stuff you can't get in the Google app store - ad-free Youtube apps - that will never be allowed on the Google app store.