Homeland Security addresses the literal security of the United States domestically as well as international facets. The rise of terrorism is unprecedented and needs to be vigilantly monitored, addressed, identified and contained by whatever means available to protect the citizens, assets and infrastructure of the country. It also includes the identification, protection and elimination of anything that would intentionally interfere or attempt to destabilize the government, its policies and operations.
It is a massively complex problem since it spans the globe, and the intelligence we gather is constantly changing and fluid, let alone the question of its validity. The nature of terrorism allows incredible flexibility and unlimited opportunities to pursue soft and hard targets of opportunity not only in the United States, but United States personnel, citizens and targets overseas/on foreign soil.
It has also expanded to work along with our allies since Homeland Security is not just a United States exclusive agency, but many countries have been forced to form their own homeland security agencies since terrorism has no geographical boundaries.