If you are going to introduce a flat tax on a good--say 5% for all members of society regardless of wealth--that tax would affect the poor more than it affects the rich, even if it is functionally the same amount of money. With this in mind, it may make sense to apply such a tax only to goods that are purchased more often by those that have the means to pay the extra tax without needing to make sacrifices for it, e.g. a luxury tax.