Obviously travel bans must help at some margin. How large a margin is debatable, but it's absurd to suggest that preventing people from coming into the country from other places with a sizable caseload won't reduce our caseload.
It can still be bad policy, and fine to argue that, for example, this is compromising our efforts to coordinate a global response by insulting our allies. (Certainly it was an obvious failure not to warn them).
But "Do travel bans work?" Of course they work; some number of fewer sick people enter your country. Do they keep the disease from ever getting here? No. But they help #flattenthecurve and we should not be debating whether arithmetic is real.
