The important thing is that mere existence of the option - and the possibility that if you tighten the screws too much, it can go from 1-in-200 to 1-in-20 overn

The important thing is that mere existence of the option - and the possibility that if you tighten the screws too much, it can go from 1-in-200 to 1-in-20 overnight - keeps the policies and pricing of the "primary" store in check. Competition is useful for consumers even if they don't switch to a competitor, because their current vendor expects that they could, and can't freely do monopolistic exploitation.