Allow sending messages while being disconnected (offline)
XEP-0198 allows you to easily have a conversation even on unreliable connections, it makes the connections basically invisible to you and your contacts.
However in Gajim if you happen to be typing, and your connection dies, when you press enter you may not be connected. Gajim pops up a dialog box that says: "Your message can not be sent until you are connected".
If your connection is very unreliable, and you are trying to have a conversation with someone, this can get very annoying. Gajim could instead add the message to the stanza queue, and it would be sent to the server when the session resumed.
But with XEP-0198 you can "send" messages while disconnected by adding them to the send queue, they will get sent to the server when the session is resumed.