So true. Guitar is one instrument that was designed with practically an invisible note structure that mainly relies on memory for position and patterns to master it. What fascinates me is the in-genius layout of that make this instrument function compared to the straight keys of a piano, for instance.
Using the circle of fifths theory in relation to guitar share some interesting connections. The notes used to formulate the order of flats/sharps are connected to A/440hz (standard tuning), combining pitch and fingering order to the frets.
It is possible to play all scales in every key using the first four frets of the guitar by the order that the notes (intervals) follow.
The notes @ the fifth fret are naturals (no #'s or b's) on all 6 strings. Notes low to high: A-D-G-C-E-A . Notice that the open string notes E-A-D-G-B-E are the perfect fifth of the 5th fret notes. Now look @ all the notes that are five frets apart ie., 12th fret and 7th fret, 9th & 4th , 7th & 2nd, etc......
This is a great way to reference important key notes and open the door to modulate between closely related keys and allow to transition smoothly with more distant ones.
The more you study the relationship between Tonic and the fifth degree it shows how perfect it is.