When concrete is placed against another structure, retaining wall, light standard, driveway or other fixed object, it is best to have a separator between them to allow each unit to move independently without binding.  There are different kinds of material used for this purpose.  They range from foam and fiber board to dimensional lumber.  The objective here is to have a separator that is softer than the concrete so as adjacent concrete surfaces move at differing rates, the expansion joint takes the brunt of the damage.  In the absence of expansion joint material, the top edge of the concrete will break apart as it grinds on the other object.