If you loved the look of your "good china" and wanted to enjoy it every day, but couldn't afford to replace broken or chipped or worn-down pieces all the time, displaying it was safer than using it every day. You could eat off the same old plate, whether it's a damaged part of the set or something else entirely, while looking at the pretty ones you're not having to wash (or even dust often, if behind glass). Then for big fancy gatherings you'd have enough nice ones for everyone.

Edit: Looking at OP's picture brings me memories of long-ago college parties, so it's a similar reminder of good times with good friends that old grandmas might get from looking at their china case.