Usually new beginners don't even know what they are and forgot to transfer them to new screen. Bonded to the original display units is a small clear plastic guide for the face camera, it stops it moving around by keeping it properly aligned relative to the hole in the screen.
When the plastic ring isn't transferred with the new display it can result in the camera showing strange blurred edges or pointing in the wrong direction or proximity sensor not working ( while you listen to phone . the screen won't light off )