What you see on the outside what you consider a phone is actually much more complicated than a screen and a body. Inside there are large parts like the camera module, 3D Touch motor, speaker, battery, etc. and smaller parts like flex cables that attach to the external parts like charging port, volume buttons, vibrate switch, power button etc. The brain of the phone or whatever you want to call it is called a logic board, which is a circuit board layered with tiny electrical components called capacitors and resistors that regulate power for different functions, metal traces made of copper or silver, sometimes gold parts anchoring antenna components, and the most important parts called the ICs or integrated circuits. Anchoring the ICs are tiny balls called solder, which form joints that connect these valuable components to each other.
If you've ever dropped your phone and cracked the glass or not cracked the glass but nothing would show on the screen anymore, one of the internal components has likely been damaged. Feel free to use your phone without a case and glass protector. But the laws of physics are against you.
You're welcome for the free lesson today. To learn more, I suggest you consult wikipedia and iPhone hardware repair forums. ifixit is a good place to start.
