Why? Because of what is largely and popularly known as Impulse Buying. When people are liquid i.e. when you have cash with you, there's always this impulse to buy something you see through the glass window of that well stocked supermarket.

The issue with this is that the items you purchase impulsively are not items you need. When you sit down later and look at all the items you have purchased, you will realize that you already have items that can be used alternatively to the new items you purchased. 

This is the major reason why we tend to have much more stuff than we ever need.