It's a bit like the shirt you bought especially for John's wedding. It sits at the back of the drawer, because you never know, one day it may come in handy... Well, if your eyesight hasn't changed much, keeping your last pair of glasses for emergencies is a pretty smart move. Because it's not just other people who forget their glasses on the train, leave them on the beach or get them run over by the neighbour's car. So, at those times we're pretty happy to have a second pair on hand.
But if you don't need them and decide to donate them, they won't be cluttering up your space anymore. And what's more, they'll make someone who really needs them happy. It's smart, and nice, to donate your glasses.