Airport and supermarket trolleys often require a returnable deposit of a £1 coin, or a plastic token the same size, to be released from their stack. The coin or token is carried in the trolley and returned when the trolley is returned to the rack.
Since it is easy to spend a coin and not have one when you need a trolley it is convenient to have plastic tokens, which you can't spend (and can give away to other folks without coins) for when you need a trolley.
