If the volume is read-write in Laserfiche, users are allowed to add documents or modify the existing ones. If the underlying disk is read-only, the Laserfiche Server wouldn't be able to service those requests properly. By making the Laserfiche volume read-only, you indicate to the Laserfiche Server that it should not allow modifications to it. Whether the disk housing said Laserfiche volume is read-only or not is irrelevant at that point.