While it helps the Kurdish people of northern Syria, it keeps a U.S. foot in the door on long range political settlements in Syria, preventing a Russian-Iran-Assad deal regarding all of Syria that could be counter to U.S. security interests in the region. The expense is minor, with the U.S. provides military a technical, supply support and intelligence role, with the Kurds and their northern Syria neighbors and allies providing the “boots on the ground”. Rather than leaving the security interests in the eastern Mediterranean to Russia and Iran, and hegemony seeking Erdogan, having our foot in the door of their plans is good for our own interests.
What are those interests?