An Initialization Vector (IV) is important because it serves to prevent predictability of encrypted messages in cryptography. Even if the IV is visible to third parties, it still provides security benefits.
If you use the same key and the same plaintext (unencrypted message) multiple times without changing the IV, you would get the same encrypted text every time. This could allow attackers to recognize patterns in the encrypted messages and deduce information about the plaintext.
An IV introduces randomness into this process. Even if the plaintext and the key remain the same, using a different IV results in a completely different encrypted text. This makes it much more difficult for attackers to recognize patterns and gain information about the plaintext or the key.
In short, an IV is important for increasing the security of encryption, even if it is visible to third parties, because it reduces the predictability of encrypted messages and makes it more difficult for attackers to recognize patterns.