What is the benefit of a unique Initialization Vector?

Question

I use AES encryption with the Rijndael class to encrypt passwords and then store it in a configuration file.

I know what is an Initialization Vector. I have read threads on SO (such as Should I use an initialization vector (IV) along with my encryption?) and the article on wikipedia.

However, there is one point I don't get: as you need the same IV used in decryption as the one used during encryption, you are forced to store the IV with the encrypted text. What is then the benefit of using a unique IV for each encryption? If an attacker gets the encrypted text, he will also get the IV.

Answer 1

Suppose you have the same message in two places, both encrypted with the same key.

The attacker finds out (somehow) what the plaintext is for one of those messages. If you've used the same IV in both cases, the attacker knows the plaintext of the other message too, because the cyphertext will be the same. If you've used a different IV (and the attacker doesn't have the key) then the second message's contents are still safe.