It's a time saver. Just looking at the guitar, one can determine what scale is needed.
To find a dorian play the ii chord of any key. From that chord gives you the actual dorian scale. The irony of modern music and today music, one can play a minor scale over a major chord or key. Apply this logic:
Dorian = a major scale one whole step below.
If your dorian root or note is G, your major scale or parent scale is F. A dorian would be G, Ab would be Gb, C would be Bb, and so forth. This logic eliminates the paper work or the constant construction or deconstruction of the rule of dorian. One has to do this 7 times for every major scale, 7 times for every harmonic minor scale, 7 times for every melodic minor scale. (21 X 12) This system helps to find the PARENT scale without extensive and tedious rote memory. It's visual. One can memorize in less than half the time. (the rule is about 30 seconds, the scales is the time one takes for the manual labor)
Hope this helps,