This is vital to help players to instantly to be able to hear and see what they are playing. Like modes for instance.....if you write down all of the modes just for one key it will help you tremedously. Honestly, you will be able to look at a piece of tabulature after a short amount of tiime doing this for instance and know exactly what scale and key the solo(s) are in. This is very useful as you can then learn very quickly how certain players gravity to certain keys and modes and intervalic idea and concepts.
It is also important to understand all of the mostly used chords in a major/minor key too. It also helps if you know some of the chord substituitions as well the to know what scales/modes work over those chords.