I guess it is for tabs kept during a single stay, rather than across multiple stays. This would then be to defend against people who 'Dine and Dash'.

Except that the rest of the world has long ago figured out the solution to this that does not involve strangers keeping my card behind a bar - they come to your table with a terminal(or you come to them), you enter the card + pin and preauthorize payment. Done - you keep your card, they can charge you if you run away, your card numbers are not compromised. Easy.