This is all based on how the RunTime API worked already (pre-dates this launch by 2 years or so). We wanted to not change too many bits.
Since function code has no listening socket/port as it were you need something (like the bootstrap script in a custom RunTime) to pull the local interface for the runtime API. That runs on the underlying worker and communicates with the API for Lambda.
Since function code has no listening socket/port as it were you need something (like the bootstrap script in a custom RunTime) to pull the local interface for the runtime API. That runs on the underlying worker and communicates with the API for Lambda.