Most recommendations I've come across regarding solder mask expansion state that it is required so that misalignment between the copper pad and the solder mask opening do not interfere with the solder connection to a mounted component.
However, nearly every default recommendation I've seen for PCB design rules includes solder mask expansions for routing vias. Tented vias are possible, but used for specific cases and not by default.
Can anyone explain why soldermask expansion on default routing vias shouldn't be zero? If the soldermask overlaps the copper by a small amount, why does that matter?
EDIT: To clarify, in my CAD package, an expansion of "zero" means that the copper pad and the soldermask opening have the SAME dimension. It doesn't mean that the soldermask opening is non-existent. To tent a via in my CAD package, I would need to either check the appropriate box or use a negative solder mask opening value.