I don’t play stringed instruments, but my fiance does. Basically, capos alter the tuning of the guitar but also offer different harmonics when moved around the fretboard which in essence allows you to have a 12 octave range. Pretty neat.
I’ve been using capo because it makes my guitar playing easy. I do not have long fingers and capo is my one way of getting around the guitar without reaching too much (LOL). Anyway, it also helps when I sing to a different key than the original…this usually helps whenever I am jamming with friends or having a gig.