Flip covers provide protection again screen scratches and, in some cases, protect against drops. However, they can sometimes cause small scratches if not used correctly.
Since metal is softer than the glass used on smartphone screens, keys and other metal objects do not pose a risk for screen scratches. Sand and some types of dirt are a threat, and flip covers keep can keep these threats away.
However, flip screens can also capture these same threats and hold them against the screen. If pressure is then applied to the case, this can result in a scratched screen. Making sure both the screen and the inside of the flip case are clean can mitigate this risk.