In short - to make it sound fuller and more harmonic. As stitch said thats how guitar and standard tuning are laid out, so why not take advantage of it and play something more than just 3 notes on 3 strings? 
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Justin states in the video (about 2 mins 20)
‘the more discoveries you make on your own the better’.
There are only five possible open position chord shapes for major chords - C, A, G, E, D.
You know that a triad must contain the 1, 3 and 5 (for minor 1, b3 and 5).
Use your knowledge of chord shapes, notes in the first three frets, triad construction etc. to find all possible triads on adjacent strings in all five of these shapes. Repeat for Am, Dm and Em chords.
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