No one else can read your instant messages.
You are assured the correspondent is who you think it is.
The messages you send do not have digital signatures that are checkable by a third party. Anyone can forge messages after a conversation to make them look like they came from you. However, during a conversation, your correspondent is assured the messages he sees are authentic and unmodified.
* Perfect forward secrecy
If you lose control of your private keys, no previous conversation is compromised.