Why does the ingress gateway require injection?

Hi,
I’ve been going through the docs trying to get a prod ready ingress gateway deployed and there’s something I don’t really understand.
Here: Istio / Installing Gateways it says that we should enable injection for the gateway pods, which, if I understand correctly, would mean an extra step for communication.
Is there some documentation as to why the gateway requires the sidecar to be injected?
Thanks.
Emil