Stub arbors are just another option in the toolbox for when an overarm is either impossible or unnecessarily inconvenient.
They're particularly useful for slitting saws or other small features that may require spacing a little from the spindle face for clearance but don't require additional support. If a formed milling cutter (i.e. concave, convex, single or double angle) needs to be angled in a vertical head, a stub arbor is perhaps your only option. Mounting the work angled instead of the cutter may be difficult or inaccessible or present other obstacles.