Why would someone ever take that challenge when they can just decline and slap the shit out of our kroxigor?
Some armies (okay so WoC) have to accept challenges, on top of that even if the enemy refuses/does not have something to challenge, the enemy unit can only attack the kroxigor unit champion (only model in B2B) and 1 of 2 situations will arise: 1) enemy fails to kill unit champion - unit champ and supporting Krox make their attacks as normal, in the following round the enemy may only do as many wounds to the unit as the unit champ has remaining (as he is still the only guy in B2B). 2) enemy kills unit champion - excess wounds do not overflow onto the unit. Meaning they can only do a max of 3 wounds. The supporting krox steps up, and spare krox moves into support. Both krox can strike so all you've really lost damage output wise is 1 attack.
That's the trick. A cheap one ofc. Also not only serves against WoC or some hero/champions that are forced to challenge. If you hit a bunker that has a mage going b2b, its common to send a challenge in order to sacrifice the champion for saving the mage at least for one round, so in the next turn you could be able to counter charge and save the mage. Loosing only the champion and the unit and mage remains safe. Having mounstrous supporting attacks can actually kill the b2b mage. Same terms with a BSB who usually has low protections and is killable. And what is more important, if you are facing average infantry troops you may win the combat or lose by 1 or 2, keeping your krox into the fray.
It's pretty tame compared to what other armies can do with it. Ogre and minotaur units with characters who hide behind command models and then make-way into combat, or the dreaded "direwolf darts" that can tie up a blood thirster indefinitely if he gets caught (undead champion regenerating shenanigans).