Silver is specified where a planter needs to read as modern without introducing another color into a scheme. It works against glass, polished concrete and pale render, and it is one of the few finishes that looks deliberate indoors and outdoors without changing character.
The useful comparison is not with metal but with grey. Matte Cement Grey is a flat mineral tone that reads as mass and sits still; Metallic Silver contains flake, so it lifts where light rakes across it and reads cooler and darker head on. Choose grey when the planter should feel built into the setting, and silver when it should catch the eye at the end of a sightline. They are not interchangeable, and on a scheme using both the difference is what does the work.
Because the finish is directional, plan for it. On a single specimen planter the shift is the point. On a long row where light changes along the length, expect the tone to vary from one end to the other, which is why a flat finish is the safer specification for a continuous run.