For AES CBC encryption, whats the importance of the IV?

Question

What is the security threat of always using all zeroes for the IV? If it allows the encrypted text to be deciphered, how could an attacker do that?

UPDATE: So then,

Answer 1

Not easily, but it does expose information about the messages, which is something that encryption tries to avoid.

Let's say you use the same key and a zero IV. Someone sniffing traffic can notice that messages start out with the same ciphertext. This gives them some information. By randomizing the IV, this information leakage goes away. More powerful attacks can be launched by knowing more about the structure of the plaintext data. Additionally, if you send many messages in such a way, an attacker could slowly learn more information about the key being used due to the reduction in variance (only the plaintext is different, instead of both the plaintext and the IV