1/ Fully vaccinated people can still be infected. So every case found allows the contact tracing teams to focus their efforts on finding all the branches of any transmission chain.
2/ Plus it would seem that the fully vaccinated can still be capable of infecting another person.
Yes, we are all well aware of this. You’re ignoring the fact that vaccination significantly reduces the risk of the person being infected. We need to stop thinking in terms of “covid zero”. It’s about managing risk.