It's a pretty common technique for exploiting Blind SQLi. You can use this as the one of the Branches in a SELECT IF to be able to determine the value of something in the DB.
Without giving too much away (it's a rather good series, and the reveal is pretty well done), it also might have to do with the book Sybil about Shirley Mason
https://www.owasp.org/index.php/Blind_SQL_Injection#Time-bas...