It’s a cool little piece of functionality that will help out developers building asynchronous apps on top of WordPress.
At first, I didn’t see the need for it. (It’s not too hard for developers to create their own script to poll the server every few seconds.) But I soon realized why an API like this would be useful. If you have 5 different plugins all with their own server polling, you are going to have 5 different hits to your server every 15 seconds. However, if they all piggyback on the Heartbeat API, those polling requests are going to be bundled so you are only hitting your server 1 time every 15 seconds. There are other benefits, but that’s the big one to me.
Let me know if you have any questions about the API and I’ll try to address them.