If a country has 10 imported COVID-19 cases a day, how much of a problem is it? Well, it depends on how many cases there currently are locally... 1/
If half of new cases are imported and half are locally acquired infections, then (assuming detection/control is same for both) we'd expect half of transmission to come from imported cases. Stopping imported cases would therefore halve the current sources of transmission. 2/
If 5% are imported and 95% are locally acquired, then preventing imported cases would have far less relative effect, because most transmission is likely to be happening from local cases, which will continue even if importations are prevented. 3/
Look at this figure (from April, nytimes.com/2020/04/09/bri…) - the ratios of imported cases across the three countries are different, which means the relative role of importations (and relative effect of preventing them) is also different. 4/ 
So the question isn't really "how much of a problem is 10 imported COVID-19 cases a day?", it's "what proportion of cases are imported, and how much overall reduction in infection will come from preventing these imported cases?" 5/5
