If you wanted to harm the company, you wouldn’t have given them notice.
Obviously, if you had malicious intent, you would have ample opportunity to act before giving notice.
They just want to make sure they did what they could in case you should in fact have malicious intent. 'Oh, we escorted him off the premises within the hour; this one is not on us...'
Many very serious meetings were then held to ensure this could never ever happen again, regardless of the cost-benefit tradeoff, because inconvenient procedures don't make headlines.
It's not about stopping targeted harm (which is covered by other security procedures) but to limit the time where the employee is at increased risk of leaking data or doing things that they normally wouldn't because the main consequence of losing their job is removed. Someone knowing they'll be gone soon can also have a negative impact on team productivity and morale.
The same usual problems with false positives and false negatives.
(also 99.9% of employees are not leaving, considering them all a risk can be expensive)