The battery can die.
An iPhone battery is good for about 500 to 1000 cycles. When it gets over 500 cycles your iPhone battery will not hold its full capacity.
The camera suddenly stops working.
A drop or a knock - even if it's not serious enough to break the glass - can cause internal damage to the camera.
Touch ID stops working.
The iPhone won't read your fingerprint to unlock.
Touchscreen doesn't swipe or tap.
A good knock can kill the digitizer even if it doesn't beak the glass. Screen replacement is usually the necessary evil/good. ;)
Unable to update to the latest iOS.
Software problems and damage or both could contribute to an iPhone being unable to update its firmware and iOS versions.
WiFi drops connection or has limited WiFi range.
Physical damage is usually the culrpit when you experience WiFi woes.
iPhone got really slow and the battery doesn't last.
Some models of iPhone will slow down when the battery isn't so good to keep it running longer without shutting down.