Because there are people like you that don't understand that 30 seconds is worth investing for $40 in instant savings. That's precisely why.
That doesn't actually answer the point _at all_.
Again, why do the retailers bother with the things? What's the benefit for _them_, to offset the loss of business from the delays caused at checkouts by people fussing with coupons (which are rarely for more than small amounts). The retailer does not get "$40 of instant savings" (nor do the customers in most cases as the vast majority of coupons are for far smaller amounts).
My best guess would be they get some sort of useful data, as the coupons let them know which kinds of media their customers are consuming and where they are hearing about the store or product from. But still hardly seems worth it. I sometimes use those from my supermarket, but I kind of resent my own compulsion to do so - not at all sure it's worth the bother, I'd rather they just lowered their prices a bit and saved me the effort.