Major scales are one of the most fundamental tools, both from a practical and theoretical perspective on the guitar. From a technical point of view, being able to play every major scale in any position on the guitar gives you great access and control over the fretboard.
From a theoretical point of view, major scales are the cornerstone for much theory relating to soloing, composition, chord construction and the formation of other scales. When you construct chords, you are actually using the notes from a major scale to do so. For example, to play an F major chord, you need to use the 1st, 3rd and 5th notes (F, A, C) of an F major scale. Even the construction of minor chords and altered chords uses major scales as its basis. For example, a Gmin7b5 chord uses the altered notes of a G major scale – 1, b3, b5, b7 (G, Bb, Db, F).