I think prints were originally sepia-toned for two reasons: First, sepia-toned
prints are more permenant than untoned prints; second, sepia-toning gives a
nice warm-tone to a print so that people look better (more like skin tones).
Some folks who hand-color B & W prints, sepia-tone them first for that reason.
BTW there are many sepia-toning formulas so I'll quallify my first statement
as applying to sulfide sepia-toning.