* Encryption
No one else can read your instant messages.
* Authentication
You are assured the correspondent is who you think it is.
* Deniability
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.