Why learn the notes on the fretboard? To apply scales and chords.
There are many ways to get to know the notes on your guitar. One way is by studying a fingerboard chart. An enjoyable, hands-on way, is by playing octaves. Octaves are simply two notes that are the same note, but different in pitch. One is twice the vibration of the other.
Octaves are two notes that are the same, yet one is half the pitch of the other.
There is an octave shape we can apply throughout the neck. By utilizing this shape we can recognize the notes on the 6th, 5th, 4th, and 3rd strings quite easily.
Here are the octaves for the 6th and 4th strings.