Some metals, such as tempered steel, titanium, and chromium can scratch glass; in addition, the rails of a phone are often aluminum, and thus can be easily cosmetically damaged by contact with something as commonplace as a set of keys, or an abrasively textured finger ring.

Further, if your phone has a finish on the glass, such as the “soft touch” glass back on the pixel 3, then the finish may be easily damaged by contact with materials that would otherwise not damage glass.

It is true that glass has improved markedly for phone screens and backs, however this does not render your phone indestructible, or infallibly resistant to damage, surface or otherwise.