The capo effectively acts as a moving nut (or as a barre finger) A capo is useful for transposing the key of a song, especially if the song contains a lot of open strings or open chords, if the song contains only bar chords, then there may be no point of using a capo as you could just as easily transpose the bar chords to the desired key Capos are particularly useful if you are intending to sing along while playing the guitar If you find you cant sing the song in a particular key, try using the capo at different frets until you find a key that you can sing in