Ah wondered where they got them from! Makes sense. Haven't tried it yet but pretty certain it can be used by existing customers too. There was a deal on yesterday for £30 Deliveroo credit for £9.99 (it's disappeared now) but people seemed to be using them fine, and if they are from Wuntu then their terms state that they can be used by new and existing customers.
Free delivery, essentially. Confusingly they also have this deal on https://foodscene.deliveroo.co.uk/terms/deliveroo-mastercard-score-2-50-off-your-order.html which gets you £2.50 off your next order when you pay with MasterCard and enter MASTERCARD as a voucher code. No idea if they stack...