We had very similar at work recently.
We all went out for lunch at a well known pizza / pasta place. The bill (for the sake of maths...) was £150 for 5 people.
Person 1 had a 25% off all mains voucher - wahey! says everyone, thank you Person 1! Bill now £112.
Everyone splits the £112 five ways, so £22.50 each.
Person 2 had a gift card for £30, can only be used at this restaurant. Person 2 says - you know what, I'm not going to come here again before the gift card expires, why don't we use that £7.50 to go towards the tip.
Thanks Person 2! says People 1 3 4 and 5, who now work out that they need to pay (incl 10% tip) around £25 each, thanks to Person 2's generosity with their remaining £7.50.
Everyone ends up paying £25 rather than £33 each.
Everyone is happy with Person 1's 25% off discount code, and Person 2's extra money towards the tip.
No one enquires why Person 2 has a gift card - maybe it was a present? Maybe they got it as they could get a discount on gift cards online, and only paid £25 for a £30 gift card? Maybe it was a Christmas bonus from work? It's just their way of paying for their share.