Why is this a Chrome thing, not an Android thing?
I get that this could happen on any OS, and the proposal is from browser maker's perspective. But what about the other side of things, an app (not necessarily browser) talking to arbitrary localhost address?
Basically any inter-process communication (IPC). https://en.wikipedia.org/wiki/Inter-process_communication . There are fancier IPC mechanisms, but none as widely supported as just sending arbitrary data over a socket. It wouldn't surprise me if e.g. this is how Chrome processes communicate with each other.