The purpose of this lesson is to show that it's possible to play something on the guitar "by ear;" that is, without any sheet music (traditional or TAB).
It may seem corny to figure out how to play the melody to "Happy Birthday" up and across the neck, but I think it can help the beginner understand 1) how the guitar works, and 2) that "music is something you hear, not see."
One of my goals as a guitar teacher is to get the student to be able to play by ear; that is, to be able to hear something in your head and play it. This is, of course, the way music was played originally.
Being able to read music is a valuable skill, but too many times I meet people who took piano lessons for several years (for example), but cannot play a note without sheet music. To me, this means they never really learned to be musicians--only typists!