I don't really know the answer...I suppose it makes some reaches easier, and is convenient. I've read a few explanations in the past that weren't very illuminating. In the end, much of the answer is.."tradition" 

(I did a google search, and kept coming up with the same post across a number of forums, so I'll pass it along. It's from a user named NoteBoat on GuitarNoise. I don't know if this is the first instance of the post)

by NoteBoat » January 10th, 2009, 5:22 am

The history of tunings isn't well documented. There are a couple major reasons for this: the guitar is easily re-tuned from one piece to the next (and in fact, had to be - more on that in a bit); and when early historians didn't record tuning information we can't go back and investigate - a guitar left alone will go out of tune, making it impossible to determine the original tuning. So what follows is largely my own speculation, but it's based on a lot of research I've done into the precursors of the modern guitar.

The guitar has a string length of roughly 25-1/2". The nearest orchestral instrument is the cello, at about 27"; it's probable that the first guitar tunings were just like the cello - because the early guitars had four strings... it would be pretty easy for any trained string musician to double on the guitar, since the cello, viola, violin, and all other orchestral strings of the day (except the double bass) were tuned in fifths. Cellos are tuned to CGDA, sounding slightly lower than our four bass strings. It's my guess that tunings in fourths date to the middle renaissance, and minstrels - the first documented folks to sing AND play at the same time - changed the tuning so they wouldn't be distracted by finger stretches.

Fretted instruments have been around for 3 millenia or so, but the frets had to be moveable - because there wasn't a a standard tempered scale. In other words, the distance between G and B (and any other pair of notes) was different depending on what key the music was in. Since stringed instruments can produce the same pitch on different strings in performance, changing the tuning wasn't a simple matter of tweaking the pegs... you actually had to move the frets a bit. These early frets were generally made of rawhide and knotted around the neck. A good shove could probably move it enough to adjust the tuning - but you had to do that with every fret. You can imagine what a pain that was!

By the late Renaissance instrument builders got the idea of adding more strings. In modern reproductions of early lutes these are often tuned to different pitches about a second apart (one string for C, the next for D, etc). It's possible (though far from certain in my mind) that this was the way lutes were tuned, but I think the reason for the tuning was to eliminate the need for moving frets - you could then play a G on one string in one set of keys, and ANOTHER G location in other keys. No matter what the actual tuning was, this development also led to the one forensic tuning tool we have - since you had to know what string to play, standard notation wouldn't suffice... and tablature was developed around the 15th century.

This doesn't solve the problem of researching tunings, because most of Europe used letter names for the pitch on each string line. That was probably neccesary, since existing music for fretted instruments seem to have used different tunings. At any rate, we can get a rough idea of guitar tuning by comparing tab from Italy (which used numbers) to tab from elsewhere in Europe (which used letter names), and cross-referencing those few pieces that were also transcribed for other instruments... my best guess is that at least some Renaissance guitars were tuned to G-C-F-A-D-G. This is a tuning of P4-P4-M3-P4-P4, but it put the major third interval between the third and fourth strings, rather than today's 2nd/3rd. But it's also clear that lutes were tuned in different ways in different places!

Anyway, that's where the major third "gap" in our tuning comes from. And the reason for it is pretty simple: it makes scale fingering easier. Without it, almost every scale fingering (even on a five-string instrument) requires a one-fret shift. Putting the major third in the mix keeps you in one position.

Things change again with the development of metal strings. Guitars* with metal strings start appearing in 17th century Italy - but we're back to four or five strings; we didn't have the ability to support the higher string tension, so we used fewer strings. The sixth string didn't pop up again for about another 150 years - and when it did, it started with gut-stringed instruments. The modern guitar is only about 150 years old, and the guitar with metal strings is only about 100 years old.

The use of the guitar as a chordal instrument is fairly recent; through the baroque era, fretted instruments played single notes or intervals. But by using a major third, once we got around to banging out full chords, the fingering was pretty friendly.

*thoughout this, the word "guitar" really means "fretted instrument" - stuff like Vivaldi's guitar concerti were actually performed on instruments like the chitarra, which are related to our instrument, but aren't technically guitars.
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