Is external travel a significant input of new vectors compared to internal travel?

It certainly helps, it’s easier to control, and has a smaller impact on most people’s lives so more easily accepted. It’s not the most effective but other factors have to be considered, including social unrest. It seems a sensible decision IMO

>it certainly helps, it’s easier to control, and has a smaller impact on most people’s lives

it has quite a lot of significant costs, in particular on the airline industry and business travel which is smaller in number but pretty important. And given that community spread is going to be a far bigger issue this ban is nonsensical.

Seems like many companies are already curtailing or banning business travel (especially international), so this might not be too much of a change.

That doesn't answer the "significant" part, I think. It's an easy decision for the reasons you mention, but that doesn't mean it has a significant impact.

Depending on how spread goes, it would be easier to ban internal travel after banning international travel than going 0 to 100% ban in one swoop. Don’t underestimate that the damage of social unrest can easily surpass the potential damage of the virus. Remember public policies deal with people with emotions, not robots.

This would make more sense if the travel ban was bidirectional. But as you said, a lot depends on how things are progressing, which we don't actually know in the US due to poor monitoring. Which is why I said it's not clear if this is a step that will make any significant impact, although to your point it might work as theater either way.