5 is one of the nine ordinary outcomes of digit reduction, and unlike 2, 4, 7, and 8, it has no master-number neighbor to worry about — there's no 55 in standard practice competing for the same digit sum. If your birth date reduces to 5, that's simply the answer.
That makes 5 a good number to use when explaining reduction to someone for the first time: no exceptions, no "well, actually," just digits summing down until one is left.
