Threat hunting shows you who is in the world already and what they are up to. This deals with the current state of the environment and reveals what risks the organization is facing.
Pen testing shows you how you could get an opponent into your setting. This illustrates the potential damage done by not hardening the infrastructure by demonstrating how different vulnerabilities could be abused or by highlighting vulnerable this practices.
Risk detection using Threat Hunting is already seen as a crucial way to help assess whether hackers have already breached the defenses of an enterprise. Beyond helping the company find out if they are under risk hunting assault, incident response teams can also be helpful.
When the incident response team is called in to deal with a security incident that has been discovered during a search, they will be better equipped to handle the situation as a significant amount of range and triage was done during the hunt.