Your device and personal data are more vulnerable to threats such as Ransomware from apps downloaded from unknown sources. Here's some additional info about downloading from unknown sources and protecting your device against Ransomware.
From a Home screen, tap Settings .
Navigate: Apps & notifications > Advanced > Special app access.
Tap Install unknown apps.
Tap the desired app (e.g., Chrome, Gmail, Drive, etc.) then tap Allow from this source to turn on or off .