In order to let Cortana track flight information for you, you need to give Cortana the flight ID. Hence, you need to book your flight first in order to have a flight ID and then tell Cortana something like "flight status PR8010" so "she" would give you relevant information about that flight, such as traffic status and departure time from your location going to the airport, currency conversion, weather to where you're going and if the flight is delayed or not.