Secondary to these comments, even though 'real' national airlines may not exist, fictional national airlines could. For example, when an airline reaches out to your airport to use its services, it could be tagged with 'national airline' for example, meaning that this is the/one of the, national airlines of the country which your airport is situated in.
Having a national airline servicing your airport could bring with it good and bad things, for example;
- they may contribute funds towards new terminals / cafes etc which heighten the experience for their Customers
- there may be custom branding on the airport to reflect the fact the national carrier services there
- they may fly proportionally more flights than other airlines, due to their potentially dominant nature in your market
Some negative examples might be;
- that due to their potential monopoly over that country, there are fewer competitors
- they may feel like they can influence your decision when it comes to airport services / additional airlines servicing your airport
There are bound to be others, but this is a start.