Data Loss Prevention (DLP) and related things, mostly. Like alerting you that you just wrote a bunch of private customer data to a world-readable bucket. Or that some random employee is downloading all of your privileged and confidential reports, which could mean their credentials have been compromised. Very very nice if you have data stored in S3 that you really want to keep secure.

DISCLAIMER: work for AWS, have met and talked with the Macie team on several occasions. Opinions on here are my own