When I grew up we went to a grocery store 5 miles away, even though the closest was only 2 blocks. Once in a while we walked to the close store, but it was so much more expensive than the farther away one that we just about paid for the entire cost of the car just on the grocery savings.

I can't control the prices of the local store, and so it is efficient to have the ability to choose other stores and thus force them to compete on price.