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Chord shapes in certain keys are easier to finger: the primary chords in G, C, and D major are generally easier to finger than many others keys. The capo can be used to change to a more difficult key while using the same shapes as an easier key. A good example: a song written in E flat can be played using “D” chord shapes by placing a capo on the first fret. A capo on the first fret alters the tuning of the guitar upward by 1/2 tone. Since E flat is a 1/2 tone above D, the key of D fingerings are now 1/2 tone higher: D becomes E flat, G becomes A flat, etc.
Another use for capos is to select different chord voicing. You could be playing a song in the key of D major using chord shapes D, G and A. I can play along by placing a capo on the second fret using chord shapes C, F and G.