Of course there's also the harmonic relationshapes--the note at the fifth fret is the fourth note in the string's scale, at the seventh is the fifth note: an E played above a barre at the fifth fret is an A; above the seventh fret, a B (the subdominant and dominant chords in the E Scale). This works for all other first position chords as well. The twelfth is the octave fret.

Another purpose of them, of course, is to help the player note and chord positions on the fingerboard (the spots are visual guides to positions near them as well as on them).

--seed