Simple, Joan served not only as Queen of Naples but also as Countess of Provence. With that in mind, a marriage alliance could, in theory...don't quote me on this, surround France to the North and South with hostile adversaries.
Let the Black Prince survive long enough, it may even lead to Naples and Provence being added to the Plantagenet crown, either through Personal Union through their child, or perhaps a division between Edward and Joan's children if they have more than one.
Now that's just the hypothetical case, I do not know how effective it would be in reality.