OK, go to Amazon, who make Alexa Cast, here:
They say this:
You are saying data flows as on the left, below:
If id did, and you switched off the phone, the music would stop. This does not happen, Amazon say so. You lose control, until the phone reconnects. This is why it works as shown on the right.
Sending the data twice is of course pointless, unless you want to use Roon via Airplay to a non-Roon device, as @timmo does, as I did before my system went Roon Ready.