Extra base in CP box?

So, in the World Eaters CP box, it comes with bases for the lord on Juggernaut, the 20 zerks and the 10 jackhals. It comes with this one extra base that's bigger than the other circular ones - what's it meant for?

So, in the World Eaters CP box, it comes with bases for the lord on Juggernaut, the 20 zerks and the 10 jackhals. It comes with this one extra base that's bigger than the other circular ones - what's it meant for?