6. Gift cards can be used as an incentive to buy more
Does anyone ever use the exact amount of a gift card? Sure, but gift cards intended as a present often have round numbers like £20 or £100 rather than £19.99 or £105.99, which might be the actual price of products while round numbers may not match any prices in your store.
Then people often get small leftover amounts to spend in the future, or the customer may aim to get something slightly more expensive than the credit amount.
Merchants can use this to their advantage by issuing gift cards of fixed amounts like £5, £10, £20, £50 and £200 only (or whichever price range is suitable). Of course, it would be better customer service to let people decide on the exact amount – it’s still likely the gift card holder spends more than that amount.