It makes sense that once the spread of the virus is under control in an area that barriers are then established to isolate that area. In practice this is simplest at a national level, because we have national governments. Nations with a federal structure, US for example, can perhaps do so at the state level. In certain cases, and dependent on local geography, these areas can then later be widened, thinking about a single Australia/New Zealand zone for example.

But closing of borders doesn't have to mean no entry, it can - and likely will - just mean quarantining. Any people coming in from an area where the virus is still rampant will need to be quarantined for a period of time (a couple of weeks is I think what China is using) to ensure they are virus-free when they enter. For goods/produce coming in from abroad, there will likely be strict decontamination procedures.

Border closure seems prudent to me. It may cause delays in physical cross-border business, and it will likely mean huge reductions in international travel of people, but in the absence of both a vaccine and - crucially - the capability and willingness to administer such a vaccine globally, then border restrictions are an obvious and sound defensive strategy.