Best is, not to use Warpriest at all. There are much better healers in the game, don't waste to much ressources in Warpriest. But that's just a side note.

To your question: Some champs are dedicated healers, revives, supporters. It's not worth increasing the damage of these champs. They need survivability (hp and def) and speed to cycle through their abilities more often and heal a second time in the fight, if necessary.

Thanks - so if I understand the basic idea, equip artifacts based on the role of the champ.

So if the are a damage dealer, and the A1 is in fact based on ATK, then equiping lost of artifacts with ATK/ATK% makes sense.

But, if the primary role is a tank, you are better off equipping lots of DEF and HP, even if the A1 damage is based on ATK.

Is that about right?

That's the basic concept, yes.

As an addition: For damage dealers, you want artifacts that increase their crit rate and crit damage as well, not only atk. 

Crit rate = chance to land a critical hit, more than 100% have no effect. 

Crit damage = extra damage you deal, if you land a critical hit. Get as much as possible for damage dealers.