It also allows for easy in-transit checking: anyone on the "secure" side of the fare gates must be in possession of a card whose last transaction was a tap-in.

It also allows for easy in-transit checking: anyone on the "secure" side of the fare gates must be in possession of a card whose last transaction was a tap-in. There's no chance of a cheeky "it wasn't me who jumped the turnstyles - it was my lookalike", and you can have roaming groups of ticket validators who board random carriages and check everyone.