Grocery stories aren't just in the business of selling people the every day basics they need, they're ultimately there to make money. That's why they use some very clever tricks to get people to buy more than they need and to buy things they don't actually need in the first place.
So how do they do it? Well, they use a lot of tricks, including pumping delicious smells like fresh baked bread into the air so you buy more impulsively, putting complimentary items like peanut butter and jelly next to one another so you always buy both at the same time, and spreading the items people buy most often throughout the store so you always have to walk through all the aisles to get what you need.
You can get hip to all these tricks by reading this interesting TopTenz list.