I know this is years late but idk i need answers
Well as long as its relevant its not much of a problem (though in this case you do know how invulnerable saves work.)
So im also new, and I noticed that Canoptek Wraiths have a 4+ save. They also have the ability Wraith Form, which gives them a 3+ invulnerable save. Is there no point in using the regular save?
It has been a few editions since the OP posted this query. In that time there have been additions, like psychic powers such as null zone which negate invulnerable saves. So while wraiths do have a better invulnerable save than armour save, there do exist weapons, powers, and abilities which would prevent wraiths from using their invulnerable save.
So most of the time, no there is no point to not use a canoptek wraiths invulnerable save; except in the cases where they cannot take them.