[Cpoll]

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.

[dwaltrip]

It looks like there are some applications. Checksum algorithms are probably the easiest to appreciate.

https://en.wikipedia.org/wiki/Digit_sum#Applications

[mafaa]

I think small random uses add up to practical value although there are those who make a religion out of its 'meaning'.

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