Last I read, it was only cosmetic feature.
Axis tanks are the only vehicles that actually gain more armour through veterancy, I personally think, at least through a commander unlock, Shermans could get this feature, through something like only Vehicle Crews can give it to them, or as part of a Hull-down sort of commander ability.
In terms of what this can do through gameplay effects, some ideas can be the typical increased HP, reduced penetration change, reduced critical hit change, or even make units repairing the vehicle less likely to take damage.
And yes, the logs were carried so they can used it to unditch vehicles that got stuck in mud and steep ravines and such.
''reduced critical hit change'' would be nice , since it is just sandbags ...
i thought that the sandbags in vcoh was to negate the first panzerfaust , but you said it was just cosmetics... that sux.