Well, the sense is that the Oyster card system moves people more efficiently round the system, which is to everyone's benefit. The more people who use the card, the quicker people are moved round the system, therefore the financial incentives exist to encourage as many people as possible to use Oyster cards.

Keep it in your wallet, jon! Or have a couple of them charged up with a fiver so you don't lose out.

Incidentally, another reason for Oyster is that it gives TfL shedloads of information about who's travelling on what journeys which allows them to plan better (although there are clearly civil liberties implications here).