Zhang Chi had previously wanted Gu Youyou to implant a demon seal in Hong Li, which had much greater advantages compared to the human slave mark.
The human slave mark had significant distance limitations; once the distance between the master and the slave was too great, the slave mark would become ineffective.
If Zhang Chi had implanted a slave mark in Hong Li, when he went into the secret realm, the slave mark wouldn't have held Hong Li for long.
Moreover, once a target had a slave mark implanted, their mental resistance would increase, making it impossible to implant a second time.
The demon seal was different; there were no distance or spatial restrictions. Once enslaved, they would be slaves for life.
Another aspect of the demon seal was its compulsiveness; when Gu Youyou issued an order, even if it went against Ao Long's heart, he could not disobey it.
Slave marks were different; back then, Zi Mian had disobeyed a few orders.