Not every musician uses a capo, only the people of the string section use it. Not even all of the string geniuses use a capo; the most common users are the guitarists.
As what Wiki says, “It is a mechanical device which clamps on the neck of a guitar to purposely raise its tone.” That is exactly true, but doesn’t completely define its science.
A capo may be just be a little guitar tool. It clamps on any of the frets along the neck of a guitar to shorten the string’s tension, thus altering its pitch. Every fret, going up the scale, is a semi tone higher. If a guitar is tuned on a standard 440, clamping a capo on the first fret makes it a note higher or is raised to sharp, E#, A#, D#, G#, B#, and E#. In a major scale, the notes are at an interval of four whole notes (C G D A) or could be (D A E B). Having such pattern, placing a capo on the fourth fret means raising the pitch higher to one whole note. Capo on the fourth fret makes C chord an easy G, and a D chord to an easy A.
One of the main uses of capo is for easy pitch transposition. There are songs which require rapid chord change, and barre chords are hard to use on a 125 to 200 16shuffle beats. The capo serves as an easy tool to convert octaved chords to a much easy playing.
Artists who several times used capo on their songs are: Plumb, with their song Real, Lisa Loeb’s Stay and I Do. If you have noticed, capo-tuned guitars are often used by female artists, specifically female singing guitarists because of their soprano voices that need higher guitar pitch than that of a standard tune. It is possible to use flats and sharps like Eb# and Ab# which is tended form a higher than normal scales. But for these female singers, it’s much easier to play the guitar while singing using easy open chords and that is made possible by a capo.
Also, a male guitarist from the band Audioslave uses the capo. One reason maybe, was that Audioslave’s vocalist Chris Cornell has a wide vocal range that can seal out two octaves or even over that. He released a solo album which he himself plays the guitar on his tours and concerts, with a capo on certain songs. Tom Morello also used capo to suspend a much higher pitch with the song, Be Yourself which Cornell also sang.
If a song starts at a base octave and goes over to second, and ends at note on the third octave, then using a capo is purposeless. You even will tune down your guitar a step down for a lower flat to start with the exact note of that base octave (Eb, Ab, Db, Gb, Bb, Eb).
A thing more, when going on a gig with just one guitar scheduled to perform three songs, and those three requires a 440, step up sharp and a step down flat, it’s an inconvenience to detune and tune. It’s alright without a capo if you’ve been a roadie tuning guitars in less than ten seconds all your life. Well, if not, better use a capo. Have your guitar tuned at the lowest your gig requires, 430 at least, or a semi-tone lower than the standard then clamp for 440 raise on the first fret for another song. Lastly, adjust a semi tone again to hit a finale of G# with an easy A.