Leather phone cases are definitely protective. They protect your phone from scratches, dings, and drops. They also absorb shock, which can help to protect your phone if you drop it. Leather is a friction material, which will prevent situations where the phone case slips out of your hand. Finally, leather will help your phone case look nice and feel soft in your hand.
No, leather is a breathable material that allows heat to escape from the phone, preventing it from overheating. However, the thickness of the leather itself will be a factor if your mobile phone has overheating issues. Choose a thin breathable leather phone case to prevent your phone from overheating.