They work for you and that's great.
Doesn’t matter how good you are with money. You don't know what's around the corner. A couple of examples, that might seem far fetched, but could happen to most anyone... And let's presume a monthly spend on an Amazon card of £200. Obviously these are made up on the spot, and whilst I hope that neither happen, I would wager that everyone knows someone that has had these experiences... Andy probably works with people who have, and Rangie, whilst it couldn't possibly happen to you, I would be very surprised if your kids don't have peers who are in that position.
Example 1.
Andy B gets loses his job and Mrs leaves in the same month.
Suddenly that couple of hundred he pays off has to wait because now he has no income and has to pay rent. Only now he fails the credit check because his borrowing outweighs his income and within three months, still unemployed he defaults.
Example 2.
Rangies son and daughter, married for a couple of years are now expecting their second. They're good at budgeting but live one moth to the next and have little saved. He is self employed and she works part time. They use second credit card for groceries and his work expenses. He gets TCV and can't work for two weeks. That Amazon payment is going straight to the bottom of the list. When he goes back to work, he is told he's no longer required. Fortunately he finds another contract quite quickly, but it's 30 miles further away and pays two months behind. The Amazon card itself isn't their biggest worry, but it will be the one that trips them up first and he will only be a couple of set backs away from losing his work vehicle.
The 'I'm alright Jack' attitude is all well and good, but you aren't the primary target of these credit facilities. The number of people who don't pay them off in full every month easily outweighs the number of people who do by the hundreds, if not thousands. And that's why they are so dangerous.
And to Andys' point, a twenty pound note in your phone case would serve much the same purpose, likewise a debit card to an account with a couple of hundred in it.
And as Mr Skrunts alluded, the premium applied to prime goods more than offsets the 'free' voucher that you may or may not get.