Why are all the strings used if you only need three notes?

When all the strings of the guitar are played at once, why does it form a harmonious chord even though only 3-4 of the strings make up the original chord?

It’s because of the voicing. “Voicing” is how chords are organized, as they sound different depending on which notes are on the bottom and which are above.

For instance if you play a hand-friendly, top-of-the-neck E major chord, you’ve got two open E strings and a finger on the second fret of the D string, so that’s three E’s! Plus your G# which you need for the 3rd and 2 Bs (the 5th), That voicing for an E major sounds so full and rich because that voicing has a bunch of overtones from playing the root note (the E). and one of the amazing, beautiful things about a guitar is that you can play the same chord in a ton of different places!  You can take advantage of the different voicing each position can offer.

Keep experimenting & have fun!

“I believe every guitar player inherently has something unique about their playing. They just have to identify what makes them different and develop it.” – B.B. King