Why do people grade Japanese phone cards?

A phone card is not built to survive. It is a sliver of printed plastic meant to live in a wallet, slide through a payphone slot, and be thrown away when the units ran out. The designs people care about are separated from damage by almost nothing: the surface scratches, the corners bend, the print fades in light, and the magnetic stripe on the back takes every knock the wallet delivers.

Grading answers three questions about an object like that. Is it genuine? Forgery was a genuine social problem in the card's own era, when altered magnetic stripes were enough of a plague that two denominations were withdrawn from use entirely, so authentication is not ceremony. Which design is it, exactly? Thousands of designs share characters and series; a holder's label records the issuer, the design and the denomination as a fixed identity. What state is it in, and can it stay that way? A sealed holder keeps the card flat, untouchable and out of the light, with its unspent or spent state, readable from the punch holes, preserved as found.

It is the same logic that puts a watercolor in a frame. The picture is the object. Grading is how the picture gets a fixed identity, a verified surface, and a case that treats it like one.