Have any developments come out of adding the digits? My impulse is to dismiss it out of hand because it only works in base-10, which in my mind leans it towards numerology instead of math.
some (n mod 9) can be found by (is congruent to) (sum of the digits mod 9) instead is the most obvious example
True only in base 10 although similar congruences exist for other bases and also involve adding the digits