Just Like in real life each customer wants to go somewhere for a holiday and you need to offer them the best price and faster route you can give them. Each Airport Demand will depend on airport you want to fly from and to so to give you a fair idea. The demand from London - Germany is High than flying from London - Scotland.
Each airport will display its demand by class.
You can even fly with all out passengers. Cargo or even both.
If you feel flying will low demand will help you win you are wrong and this would result in you having less sales than having more sales via a big demand.