International flights
Flying between secondary airports on international flights can save you money, even if you have to start abroad to take advantage of cheap offers.

E.g., you want to fly from Brussels (Belgium) to London (UK). The simplest booking, but not necessarily the cheapest, is to book a flight from Brussels (BRU) to London Heathrow (LHR). A cheaper alternative could be to fly from Brussels – Charleroi (CRL) to either London – Luton (LTN), London – Gatwick (LGW), or London – Stansted (STN).

Or, you want to fly from Amsterdam (Netherlands) to Athens (Greece). If you are willing to start your flight in Brussels (Belgium), Paris (France), or London (UK), you may find that local travel agents in these countries offer much better deals than the ones in your country of residence, even if you include the costs for traveling to the airport of departure by plane or train.

Intercontinental flights
The opportunities for cost savings are basically a combination of the examples given under National and International flights. E.g., if you would like to travel from Phoenix (AZ) to Nice (France), one may find very cheap offers from New York to Nice or to London (UK) and the total airfare, including the airfare between Phoenix and New York and/or between London and Nice, may still be cheaper than booking a flight from Phoenix to Nice (including the “standard” connecting flights).