identifiers vs "to" in transactional message

Seems like it is compulsory to specify “to” address when sending a transactional message:

request='
{
  "transactional_message_id": "demo_transactional_message",
  "message_data": {
    "inviteLink": "your_invite_link"
  },
  "identifiers": {
    "id": "adc304c4-255b-42d3-bc3c-49..."
  }
}
'

echo $request | curl -v https://api.customer.io/v1/send/email \
-H 'Content-Type: application/json' \
-H 'Authorization: Bearer …...' \
-d @-

gives me:

{
  "meta": {
    "error": "Missing required field: \"to\""
  }

If I have to specify “to” address in the email, why do I need to specify the identifier? I was expecting that if I specify identifier (id of the person), it should automatically send the message to the email of the person associated with the id.

Can you please help me understand this?

Thanks,

Mayuresh