It is said that archery is a two limb venture. I can speak confidently about shooting on "the outside" of the bow - yet! - but, when I want to hit a mark, I look at the mark, and as my arms rase, in my head there is a series or "not now's" until "a suitable "now". My bow hand, knows well the feel and weight of the shaft, the power of my bows limbs and what it takes to push the shaft into the target. Arrow spur allows my bow hand to get invovled in the aiming and the power behind my shaft. In other words, the spur, gives me sort of double the ability to control the flight of my shaft than if I were to reley on my bow arms shoulder alone, as the case in most archery practice today, where the archer aims and let's the limbs "do what they do". I draw my string arm and thrust the shaft, using the bow with my bow hand.
I am of the position that said technique was common to many of the "archery dependant" individuals in our history.
To finish, I believe it is knowledge of tecniqus like "the spur" that lead to bow designs with wide brace heights, and the unusual Japanes keyudo bow (which is not commonly being used today as the design was originally intended), but I have not had the opportunity to test that belief.