Ping a colleague or get quick feedback without starting yet another email thread.
One important argument for a social intranet is that it reduces the amount of email that is sent forth and back. It achieves that using different approaches, one of them is the messages app. If you have a quick question on a document, want to clarify a question or just check something that blocks you in going forward, messages is a great way of solving that.
Where a phone call is too intrusive and email is too slow, messages fills the gap. Your colleague is not interrupted if not ready to answer. And you can skip all the usual courtesy of an email - just ask your question. And if needed, you can ask followup question 2 and 3 and be good to work again.
In the end it is all about providing meaningful tools to do day to days work. People will pick what suits them best.