@mahtan it's not explained explicitly in the manhua chapter but like
@veggklokken said, it's to ensure loyalty.
iirc in the novel it's explained better n fall somewhere in the lines of stabilizer between master disciple and it's safe, restrictions r given to both sides: the master wont harm the disciple and the disciple wont harm the master... so yea it's pretty much to deter betrayal for both sides n also maybe improve relationships between the tiny world group so they could probably support each other in the future (eg. the netherworld master getting support from others while confronting that invader that demands yu yan from before) altho they're in different sects