There are many reasons for razing eg. if you are in a province with
several structures and expect an attack you may want to move all your
forces into the largest and best fortified structure, but raze all
the others to prevent your opponent from getting fortifications for
his own use. A level 1 or level 0 use for construction would be
De-construction should be quite fast compared with construction (4x
as fast ?) and yield back half(?) of the stones originally used.