If you book a flight, it may push the number of passengers past a critical threshold, and the airline decides not to cancel the flight.
Or, if you don't book the flight, someone else who would have otherwise flown on another flight will be able to book that seat, and the other flight could get canceled.
Or, if some flights are often almost full, your booking may push the demand past a critical threshold, and the airline may choose to schedule another flight. And because there are now more flights, flying that route will be cheaper and more convenient, which may attract new passengers.
If you only fly a few times in your lifetime, you can plausibly say that those planes would have flown anyway. If the number of flights is much greater than the average number of passengers in a plane, your actions have almost certainly increased the number of flights flown.