I've never used Car Play either, but I have used Android Auto and I'll try to explain what's so good about it.
Right now, the primary benefits of Android Auto are the maps/navigation/traffic and Google Now integration. The most obvious advantage to using Google Navigation (or an alternative such as Waze) is that the maps are always up to date. No need to go to your dealer and spend a couple hundred bucks every few years to update your database. Also, road construction information is always current. Another obvious advantage is that the crowd-sourced traffic data you get in Google Navigation or Waze is much more accurate and timely than what you get over XM or HD radio. I deal with traffic jams every day on my commute and on almost every trip we take, and using the Google Nav app with Waze data or the using Waze app are the only ways to get true real-time traffic data. Traffic data on XM is typically imprecise and 1-2 hours old, so I find it to be pretty useless except in certain holiday weekend traffic situations where the same traffic patterns persist for hours on end. And one thing that stupefies me about auto infotainment systems is when they make the traffic display a separate app from the nav, which isn't terribly useful. Not to mention, I find the quality and accuracy of driving directions from Google Navigation and Waze to be a cut above anything else I've used. Sometimes far above.
Another advantage of using smartphone/Car Play/Android Auto for navigation is access to the internet. While traveling as a family, my wife and I frequently use the internet on our phones to search for a place to eat, a good place to stop and stretch the legs, a gas station, etc. Auto nav databases have some of these places, but they are never comprehensive enough that I would use them over using my phone. And when I find a place via internet search on my phone, I can jump right from there into maps and nav. With a car nav system, I have to look up an address on the phone and then type it into the nav system.
Another aspect to data entry is that car navigation systems lock out most inputs while the car is moving, which means that your primary means to interact with them on the road is through voice command. That makes having Google Now (and presumably Siri on Car Play) really valuable, because as search agents/assistants they are a lot better than your car's infotainment system.
Finally, the really big deal with Android Auto and Car Play is that they will keep improving over time. Up until now, when you buy a car you're basically stuck with the functionality of the infotainment system as it was when you bought it, for the life of the vehicle. My wife and I have kept our last three vehicles for 7, 10, and 12 years. That is a very long time in terms of Android and iOS development. I not going to claim that Android Auto and Car Play are going to make your infotainment system future-proof, but you're certain to see a steady stream of app upgrades that keep making it better, and you're not at the mercy of the car manufacturers who want obsolescence to drive the next sale.