1. A not very strategic but understandable US plan to deny the oil to both Assad and ISIS.
2. Trump's insane fantasy that the US can take the oil for itself. telegraph.co.uk/news/2019/10/2…
ISIS once made $40m/month from oil and the money could help fund the jihadists' resurgence. Assad could use the money to rebuild his shattered state and ease his own dependence on smuggled Iranianian oil.
Denying it to ISIS would be in service of the strategic goal of keeping the jihadists down. Denying it to Assad makes less sense strategically but I guess gives the US a little leverage.
Long before he was president he talked about how the US should be take Iraq's oil as a price for overthrowing Saddam.
"We've secured the oil, and, therefore, a small number of US troops will remain in the area...we'll be deciding what we're going to do with it in the future."