When I first joined my high school jazz band, it was a humbling experience. I knew my major scales and modes, but only with the roots on the E and/or A string. This worked fine for playing pop music, but the way jazz progressions were always changing chords with each measure, my hands were constantly getting lost.
If I was playing a C major line in eighth position, how did I switch to a Bb mixolydian scale without jumping my hand up or down and making the improvisational idea totally disjointed ? I could never understand how jazz guitarists could keep their ideas going as the chords changed from moment to moment. And how did players like Joe Pass know how to run an improvised line right into a chord voicing? Additionally, as I progressed to the higher registers of the guitar, I could never tell where I was in the scale anymore. It seemed impossible!