If you're prone to dropping phones, or used to tossing them on tables and rough surfaces, then protectors with thick rubber corners are recommended (unless the phone falls screen-first really hard onto an object).
If you're prone to jumbling a hell lot of items in a pocket where the phone is located, you need a leather casing or a casing that acts as a secondary protection to your phone's screen... it prevents keys and other hard objects from tearing at the screen protector (if you accidentally stored the phone with the screen facing your body in your pocket) and depending on how pressed those objects are with your phone; maybe scratch the surface of the phone's screen.
If you're looking for leather protective casings, look for those that cover corners. My brother bought a leather casing whose top corners didn't have protection; he accidentally dropped his phone one day... the magnet fastener slided a little and exposed the phone's top corners to ground impact.
If you're awfully careful with your phone and most likely wont be encountering those issues above, you probably wont need any.
