What is a Capo?
A capo is a device that clamps onto a guitar neck to shorten the string length. Guitar capos come in many shapes and sizes (and incredibly easy to use), but all have the same function: to shorten the vibrating section of the strings.
To Change The Key
The largest population using guitar capos are players who also sing, or accompany singers. You will see guitar capos used a lot by acoustic guitar players in this context.
Using a capo, a guitar player can play any chord progression he/she already knows, but have those chords sound in different keys. The key of a song just depends on where they put the capo. This simple action makes it incredibly simple to change keys.
Additionally, the guitar capo can cut the need for bar chords or playing in less-familiar locations on the guitar neck.
Brighten the Sound-Help the guitar cut through ambient noise
This can help the guitar be more easily heard among all the dancing and clapping in flamenco music (And various other types of Spanish Guitar Music). Outside flamenco, other guitarists use capos for this same purpose: to help the guitar “cut through” ambient noise.
To Get A Specific Sound
In some pieces, a composer will call for a classical guitar capo. This is usually to get a specific effect.
They may want the guitar to sound like a different instrument, such as a lute or mandolin. Or they may want the sound of pieces to contrast with each other in a suite or collection. Some pieces create unique sounds by capoing only some of the strings or altering the capo in some way . Some composers take this idea one step further by combining alternate tunings with a guitar capo.