This lets you do so many cool things, even joining across disparate services:

select
  aws.name aws_user_name,
  slack.id as slack_user_id,
  slack.display_name as slack_name
from
  aws_iam_user as aws,
  slack_user as slack
where
  aws.name = slack.email;

+--------------------------+---------------+------------+
|     aws_user_name        | slack_user_id | slack_name |
+--------------------------+---------------+------------+
| dwight@dundermifflin.com | U2EMB8HLP     | dwight     |
| jim@dundermifflin.com    | U02HE4Z7E     | jim        |
+--------------------------+---------------+------------+

Useful for compliance, security, ops, and cost management.