If you just use it for music and are happy with bluetooth, it's understandable why you don't see the benefit. Heck, do you even need a screen at that point (barring a backup camera)?

For maps, having up to date maps is an enormous relief compared to relying on the outdated maps that came with your car (at least in my experience).

> When I tried Android Auto it actually took over my car's infotainment system

Yes. I prefer it that way. The car's default infotainment system might as well not exist for all I care, since I prefer the Carplay experience.

> and made my phone slightly slower having to drive another screen?

I haven't noticed this problem.