In many cars, you can only connect your Android smartphone via cable in order to be able to use Android in your car (Google Maps, Spotify, etc.). It is important that your car must support Android Auto, otherwise this adapter won't help you at all.
What the adapter does: your smartphone automatically connects wirelessly to this adapter and you no longer need to connect your phone to the car with the cable to be able to use Android in your car.

It allows you to connect Android Auto via Bluetooth instead of the USB cable, but it doesn't add Android Auto if it isn't already supported by the car and the phone. It is useful if the car is compatible with Android Auto via USB only.