In singular it used to represent a cherished ideal of comfort and familiarity.If a room or a desk is in apple pie order,it is neat and tidy and everything is where it should be.If you say that something is as American as apple pie ,you mean that it is typically American.

People often describe others as "nice as apple pie". It conveys that a person is good, wholesome and without guile.

I think it is how Americans view life (or viewed) life in a golden age. A symbol of good and wholesome. It makes me always think of my mother baking in the kitchen. The good life.