To me it seems like a way to fight Putin without starting WWIII. If the US fought him directly, a lot of countries would have to choose sides. This way countries can choose sides but only have 2 countries fight it out.
Plus, there’s probably a long game at play. If Russia just took over Ukraine, where does that stop? What message does that send? China taking Taiwan and Hong Kong? Russia taking over more countries. After while Russia and China and allies gain power and influence above the US and its allies? What kind of control could they then exert with the resources they control that we’re reliant on?
I don’t know if that’s why, just my initial thoughts.