A real good question.
There are two trains of thoughts here in regards to a mop. One is flavor and the other is texture of the outside meat of the rib. The flavor part is much less important than the texture. More can be done with flavor with the wood smoke and prerub than the mop can add. So basically the mop reduces the dryness of the surface meat and that is its primary function although most mops are flavor enhanced; mine are.
A mop is much more important on a wood burner than an electric or charcoal unit.
Randy.