Walkers and Tennis Balls

I'm sitting at lunch this afternoon at the nearby Whataburger and the elderly lady sitting across from me has a walker. Now, it's not uncommon to see someone with a walker nor is it uncommon to see that same person with tennis balls on the back 2 legs of the walker. So my question is this, if nearly every walker I see has retrofitted tennis balls placed on the back 2 legs to make it slide across the floor easier shouldn't the manufacturer ship them with a set of Penns? Maybe a coupon to walmart for a free can of Wilson Prostaffs?
What other everyday items do you know of that are often accompanied by some other everyday item in order to make them more useful?