All voices are different, so the original keys of some songs may not be a fit for every singer. Capos come in handy when you have to find a new key of a song for a singer. Using a capo will allow you to play the song in the new key without having to learn entirely new chords. A guitar capo can do much more than simply change the key of a song, it can create an entirely different sonic experience. For example, playing in the key of A with and without a capo will have entirely different feels even though you're playing in the same key, so experiment with your capo to hear the differences. Essentially, you'll be suing different voicing to play the same chords, which produce entirely different sounds.
Find the best key for a singer. Use a capo to get a different timbre.