-
I’m not an expert but here is how is was explained to me.
You can use any of the 5 shapes (C A G E D) to play any major chord. Each shape has a root note defined on a single string. The E shape is on E string, string 6; The G shape root is also on the 6th string; the A shape is on A string, string 5; etc. By sliding that shape up and down the string you can play all 12 notes with that one shape.
That being said 90% of the time you will use either the E shape or A shape. I believe the reason for this is because the other shapes are finger twisters and more difficult to play. We are at the limit of my knowledge here so there may be uses for the other shapes.
As to why we would switch between the E and A shapes when we could play every chord with just one of them, there are 2 main reasons. One is to keep your hand in smaller area of the fret board instead of sliding up and down 12 frets. The other is to stay in the same octave range or rather have similar voicings.
Take all of the above with a grain of salt. I could be completely wrong. 🙂