Android Auto is not cheap or relatively easy to get into your car if it's not already built in. With the cheapest third-party head units costing around $400, and that's not including the labor to have someone install it or the other parts that are needed. For many, it's much cheaper to just mount their smartphone in the car and use Android as normal. So what are the advantages of using Android Auto over a mounted smartphone? Well let's explore that, shall we?
With Android Auto you get a larger interface - unless you are mounting a tablet in the car - which makes it easier to control Android Auto while you're driving without taking your eyes off of the road. In fact, that's the whole reason behind it. A larger screen, and slightly altered OS means that you have larger targets to touch. Making it much easier. Android Auto also keeps your phone quiet. We have all gotten the urge to pick up our smartphone when we hear a notification come through. But Android Auto only shows you messages, which it will read to you and you can respond with your voice.
However with a mounted smartphone, the only thing you're really paying for is a mount and maybe a car charger if you want to top up in the car. Seeing as you still have navigation thanks to Google Maps, still have music, and everything else Android Auto can do. So it's a much more cost efficient way of using Android in the car. But with a smaller screen, it means your focus is going to be on the phone rather than the road in front of you. Which can cause you to get into more accidents. Unlike Android Auto, having your phone mounted doesn't quiet the notifications from your phone. And you'll want to check and see what it was, trust me we all do that.
So there are some advantages to having Android Auto over just mounting your smartphone in the car, but ultimately the choice is up to you. We'd still recommend Android Auto as it is a bit more robust and better at keeping your eyes on the road, where they should be.