They are a corporation just like any other with profit/losses, etc.
If your country has a disaster like an earthquake, nuclear attack, etc., your country will be able to receive supplies, providing the country sending them has cargo planes, via air. You can also send aid to one of your countries in case of disaster (if you have cargo planes).
This is especially useful if your empire has more than one country. If you are a member of a federation, they may choose to send help also.
Disaster relief also can land at military airports, so you may choose not to invest in an air transport corp. It's up to you.
Hope that helps.