Well, it would've decreased the chances of a more infectious strain becoming widespread in the US if it had been implemented months ago. It could decrease the chances of worse strains that might originate outside the US becoming established moving forward.
Sure they can, but reducing the geographic spread would reduce the chances of those randomly occurring variants spreading quickly around the world. I would expect other countries to require testing of international flights for the same reasons.
The US is performing badly when it comes to controlling the virus, but it could still get significantly worse.
