Why is the "intermediate" challenge needed in Bluetooth ECDH since the "real" verification is performed at the end with code comparison?

Why is the "intermediate" challenge needed in Bluetooth ECDH since the "real" verification is performed at the end with code comparison?

Why is step 4 needed? What does it protect in terms of security? Doesn't the protection arrives from the last step so when Va and Vb (so called TK, Temporary Keys) are compared?

Other thing: I read somewhere that Cb is sent immediately and before Nb, so that attacker cannot compute a valid Cb (because attacker wouldn't have the Nb). But even if Cb was sent after Nb, what could have been the attack/weakness?

If you want, I'd like a example of an attack where 1)Cb protects from such attack, and 2)the fact we send Cb before Nb protect from such attack.