The voucher is just a way to reduce the fiat withdrawing. The situation is that some people sold BTC and withdraw fiat from the exchange, but those who want to buy bitcoins cannot deposit the fiat. How about the latter give money to the former and keep the money in the exchange? That's the reason why voucher is proposed. Those who want to withdraw fiat generate vouchers instead and sell them to the one who want to deposit, and then the latter use the voucher to deposit.

You can see that there's no fiat increase by using vouchers. It's just a way to reduce the amount of fiat withdrawn from the exchange.