Sanctified melee weapons or ammunition says it makes the damage 'Holy' and that this has certain effects as dictated in creature entries. Does anybody know any examples of this?

The most common thing this deals with is the Daemonic trait, which doubles the creature's toughness against all attacks except certain types... which includes weapons that deal holy damage.

There are, I believe, other entries with things like a Regeneration trait that can heal all damage except that inflicted with a holy/sanctified weapon.

Also, some enemies such as warp animated zombies have the "unhallowed" trait. Those enemies take double damage when holy damage is inflicted.