Sterling silver tarnishes. This is a property of the metal, not a fault in the piece, and it is the reason we chose it. A brushed surface slowly darkens in the places a polished one stays bright, and the piece begins to carry a record of how it has actually been worn.
A plated piece cannot do this. As its coating wears, a different metal appears underneath and the object simply looks damaged. Solid silver only changes. It can be cleaned back to its original state at any point, or left to keep its patina — and unlike a coating, it can be professionally restored for as long as you own it.