Shoulder pads probably help the jacket drape better off the shoulders. I know mine aren't exactly chiseled out of marble.
the shape of the suit should relate to the shape of the man's body. many tailors will tell you that if you have one shoulder higher than the other, they can add more padding on one side for symmetry. this is the lazy way of doing it. the adjustment should be made when creating the pattern. if you have small shoulders, like i do, that is all the more reason to avoid shoulder pads. they become like make-up; you'll look good with your jacket on, but when you take it off you look bad, like you were wearing a disguise. my custom suits have been made without padding. the tailor may put something in there to aid construction, but if he does, it certainly doesn't "pad" the shoulder. my favorite suit is still the isaia i tried on a few months ago (which i could not afford)-no padding and the shoulder seam was on top of the shoulder, rather than jetting outward. guys always say "i like my suit to look this way or that way" but to me the best ones are the those that make you look like you.