Peter (segment's CEO) had a great answer to this in the thread https://www.quora.com/What-is-the-advantage-of-using-Segment...
TO summarize and highlight the main reasons
1) With GTM you still have to write code. Specifically, you need to figure out how to send events to a destination following their API and JS library. With Segment and Rudder, you just call a couple of generic functions identify, track, page etc.
2) You often want a copy of the events in your own S3 Or Redshift or Snowflake. GTM doesn't help you there but Rudder/Segment can.
3) We have some features like Replay etc which lets you send historic events to a new destination. Say you signed up for a new Analytics tool and want to send all your historic events - Replay helps there
4) Finally, Segment has this product called personas which lets you create audiences (e.g give me all people who have done X but not Y and then send that to MailChimp for emailing). We too are working on it.