There are manufacturer coupons, which might give an amount off, but it could be for any of their “toothpaste”. For their smallest tubes the amount off could be over half the sale price.

Then a lot of stores do “double coupons” where they match the manufacturer’s discount. And then the item might be on sale anyway, so there’s an extra discount.

So now you have a situation where the coupon and store doubling of it add up to more than the sale price, so you get money back. People (ab)use this to print out a hundred coupons and buy 100 items to clock up “free” money that they then use to offset against the rest of the shopping, which they are also targeting with coupons.

A lot of stores have taken measures to limit this in recent years - some have a limit of, say, 20 coupons per customer and/or small print that says the coupons cannot reduce the sales price below zero.