One might object that forgetting to add <whatever> to the list is really not a terribly large problem, and you would be right- but $5 is also not a terribly large sum.
For me personally, there's usually a gap in time between when I notice <whatever> needs to be replenished and when I actually do it. Hence, the possibility to forget. Having the button co-located with the item in question essentially eliminates that problem. And $5 is worth it for me not to have to make a special trip to the store when I really DO need <whatever>.
