Spaw is right. But you not only lose the effect of the nut on open strings, the action of the guitar is slso lowered (like replacing the nut by a nut the height of a fret) and this takes away some brilliance from the instrument.
I sometimes use a capo just to lower the action and tension when I accompany myself singing. I tune down a tone and capo the second fret. Since I tend to sing softly (shy, yaknow) and play loud, this helps keep my voice from being drown out.