Setting a Default Prefix allows you to choose one location/prefix which the system will always try to apply at best effort if any phone numbers provided are not submitted in the full international E164 phone number format and it's unclear to which country the number belongs to.
Example for Cyprus:
+357 is the international prefix for Cyprus.
+357 99 123456 could be a valid Cyprus number in the international format.
99 123456 is a valid Cyprus number but without the international prefix for Cyprus. If you provide it in this short/national format, our system cannot recognize which country it belongs to - except if you have defined Cyprus as your default prefix.
We generally recommend to make sure you always provide the full format to prevent any issues. Anyhow, setting the default prefix can help make this more convenient for you - especially if all or most of your phone numbers come from mainly one country.
If your main destination is Korea but from time to time you always have numbers from other countries in your list of subscribers, then you would still benefit from setting your Default Prix to Korea because for your main destinamtion the system will always try to apply the country prefix in cases where numbers were provided without a clear prefix befor discarding them as invalid numbers.