You must distinguish between polish and smoothness. Smoothness—smoothing your surface—is the last (the zero) of various degrees of roughness, and it has its place in all figures, representational and abstract. You might say that all sculpture is a variation of smooth and rough surfaces.
Another thing altogether is polish. Marble figures have usually but not always been polished, though that is a very unsculptural thing to do to them. Why?
Sculpture is a thing of shadows; shadows define the figure. The eye uses them to understand it. Ultimately it isn’t the shapes themselves that the sculptor must “get right” but the shadows they cast. For, remember: sculpture is not simply a copied thing—a head, a torso—but an illusion, a suggestion of that thing—that thing with life. If the shape is “right” (the eyelid, say, has the proper curve of an eyelid) but the shadow it casts is unclear or conflicts with the general expression the sculptor wanted, the bust fails.
Now, polish eliminates shadows. It throws back the light and levels the sculpted forms. It produces not just glare but a general confusion of the surface.
Then why did all the great sculptors polish?
Mostly they were following a convention. People want their statue to be not just a sort of three-dimensional picture but also a “feelie”. There’s nothing quite like shiny marble to run your hand over. And a shiny statue is a more cheerful and decorative thing to have around than a dull one.
Not all sculptors polish every one of their statues, or the whole surface of any single one. The young Michelangelo polished his Pietà completely, but as he matured he used polish selectively and to better effect in, for example, the big figures in the Medici tomb. Look at the Slaves in the Louvre and decide whether it isn’t a pity that all their sensitive anatomy is partially erased by the light that slides around their surface. Who would say that the unpolished Giants in the Accademia are not more successful, more memorable (though less cuddly, it’s true)? Ultimately, the sculptor, just like the painter, must decide in each case, with each work, in each part of the work, whether a shine will enhance it, and how much shine. His polish can be compared to the varnish, high gloss, low gloss,
a painter might apply to his oil painting for the same reason.