The idea is that if you've "finished" with your Oyster card you can drop it into this box and your deposit & the remaining amount will go to The Railway Children Charity. The charity supports street children living alone and at risk on the streets. It helps children all over the world, many of whom live in and around railway stations.
Liverpool Street is a good choice for this as it's used by a lot of tourists flying to Stansted Airport who may have bought an Oyster card for their stay and now have no further need for it.
Plus there may be some City types who've travelled down to London for business, are expensing their travel & don't think it's a big deal giving the balance & deposit to charity. I'd guess that most regular commuters wouldn't want to give their Oyster card away.
If anyone knows anything more about this service, I'd be interested in knowing how many cards are donated and how much has been raised for the charity.