The choice is between a button and ordering the product on your smartphone in ten seconds, or picking up the product at the grocery store the next time you're there.The choice is between having to remember to buy the product next time you go shopping, having to remember to order the product next time you are in front of the proper device, or not having to remember a thing.
Being able to move a task to 'done' as soon as it is recognized is a savings in cognitive load far more valuable than any time savings.
Then again, I am the type of person who frequently says "might as well do it now before I forget." Younger, more single people might not see the value in this sort of thing.