Besides the decreased surface area and lubricant retention for friction, a textured surface (by whatever method) would retain oil for rust prevention. Manufacturing metal parts which will make contact with other parts with given tolerances still leaves a lot of guess work of how the pieces will fit and bear against each other in use. If a machinist only textured the area he thought would rub against another in use, the textured area would look strange, thus texture the entire area or part and make it more aesthetic. Imagine how a bolt would look with only the areas that rubbed jeweled.
