"What is the CFO Seminar (Philippines) and why is it required?"
The CFO (Commission on Filipinos Overseas) Seminar is a Philippine government requirement for Filipinos leaving the country. It's partially for their internal counting of people expatriating, but also it gives a crash-course on a few things Filipinos should know when emigrating out of the country. Some of these things include how to dial "911", and how to avoid being scammed or taken advantage of. It aims to target the exploitation and abuse of Filipinos who might be taken advantage of by rogue employers or human traffickers.
When a Filipino tries to leave the country, they go through customs and if the CFO stamp is not in their passport, they will not be allowed to exit the country. You should get this stamp long before your flight. It is a requirement of the Philippine government, not the US government.