The easiest way to get good at playing the guitar is to memorise a bunch of chords. A better way is to learn a bunch of chords in the keys that they most likely appear in. Starting with the natural notes (those that have no sharps or flats - or the white keys on a piano), we have the scale of C Major. Learning the chords of C Major helps us predict what chords are in the song or piece of musc we are learning.