I use one when playing for the Church. It helps when having to play in some of the odd keys just place the capo in the correct fret and you can play your open chords but still stay in key.

Sometimes you only have about 30-45 minutes to learn all of the 4-6 songs that they will play at the service that day and you don't have allot of time to transpose and find the chord that will work best in each song in that amount of time.