The reason to use CarPlay is that my phone already knows everything about me. And the car does not.
If there's an appointment in my calendar coming up, the car will automatically set up directions to that appointment. It'll be sitting there, ready to go in the maps app as soon as I get into the car.
Even without an appointment, if I tend to go to the same cafe every Thursday morning, my phone notices that and offers to navigate there next Thursday.
All the music, audiobooks, and podcasts I've downloaded are already on my phone. And the phone knows where I've left off on all of them. Start any of them up on CarPlay, and it seamlessly starts in the right spot. And when I finish, my phone now knows where I stopped in the car, so I can continue from where I left off on my phone later.
If I want to make a new appointment or set a reminder while I'm driving, I just invoke Siri and ask it to set it all up for me. And now it's on my phone, where I need it to be.
And since I have two cars, regardless of which one I take, I get all these same integrated features, presented in a consistent fashion.
The car as a siloed computer has none of these advantages. It constantly needs to be told where I want to go and what I want to listen to. And anything I tell it stays in the car.
There is no in-car computer system than can compete with the integration that comes from CarPlay.