The main purpose of the US Treasury Department for OFAC's extraterritorial reach is to:

The Office of Foreign Assets Control (OFAC) is a department of the US Treasury that administers and enforces economic and trade sanctions imposed by the US against countries and groups of individuals, such as terrorists and narcotics traffickers. The sanctions can be either comprehensive or selective, using the blocking of assets and trade restrictions to accomplish foreign policy and national security goals. The main purpose of the US Treasury Department for OFAC’s extraterritorial reach is to apply these sanctions to any person or entity that is subject to US jurisdiction, or that engages in transactions involving US persons, US dollars, or US goods or services. This way, the US can exert pressure on the targets of the sanctions and prevent them from accessing the US financial system or market. OFAC’s extraterritorial reach is authorized by Congressional legislation or presidential emergency powers, and is often based on UN Security Council resolutions or multilateral agreements.

Office of Foreign Assets Control (OFAC): Definition, Sanctions

About OFAC | Office of Foreign Assets Control

OFAC’s Extraterritorial Reach: Purpose of the US Treasury Department