Note that your message is not sent directly to the other device. In fact your message is sent on some nodes of the DHT and your contact will retrieve the message on this node.
It helps anonymizing text message without revealing the IPs. I didn’t know Jami worked that way. But I suppose it can’t use the DHT when sending attachments or for audio or video calls. Imagine sending a few TB file as attachment and crashing users with Jami on their phones…
It might explain why I receive multiple notifications on my phone when I receive an attachment 
An option for a direct IP for true P2P exchange with trusted contact is a good idea.