I just like re-rolls, and that is effectively a re-roll of the dice. Always liked them better than a bigger dice, or a flat bonus.

And yes, the Anti-Magic Field affects the Primewarden as well, meaning no magic items, buffs or magical healing. Getting it at 16th level blocks them off from any of the big permanent stat bonuses (Limited Wish). That means that for boosting stats, they might get a few small boosts with low-level tomes, but nothing big.

It also means they're very limited for healing at high levels, and effectively cannot be buffed whatsoever.And, if they want the anti-magic field to do the most good by affecting some monsters, they need to be a little bit away from the party, lest they negate THEIR magic.

Most of their abilities, really, are concerned with mitigating the disadvantage of having no magic available to them.