A fingerprint on the front means you can unlock the phone when it laying beside you on a table without picking it up. It’s also easier to reach for some people depending on the size of the phone and the specific manner in which you hold it.
Placing the fingerprint on the back means it’s easier to hit when you take the phone out of your pocket - it normally falls just under your index Cheap Phone finger. Removing it from the front also lets manufacturers remove the home button and trim the bottom bezel. Finally fingerprint swipe gestures (those are still relatively rare) are easier to perform.
The obvious downside on the rear-mounted sensor is that you can’t use it when the phone is laying on a table or a desk. It can also contribute to thickness or make the design of the back a little too busy if the camera setup is more complex China Phone than usual (LG G5 says hi).
The fingerprint on the side is always easy to reach and always there no matter if the phone is in your pocket or laying beside you. However performing any gestures with it is bound to be super awkward and it could explain why Sony isn’t doing those. More important downside is that the side placement means you need to have some extra bezel next to your screen for the module to fit. This doesn’t really go great with the current consumer demands for larger screens in bodies as narrow as possible so they are still manageable.