If your trip has a connection, the onward ticket question starts with whether you are entering the country or simply changing planes. An airside transit and a two-night stopover may appear almost identical on a flight-search screen, but they create very different travel plans. Before buying a temporary flight reservation, identify the country where you will actually pass through immigration and the flight that takes you out of it.
Consider a journey from London to Bali via Singapore. You might spend ninety minutes at Changi and never enter Singapore. Or you might book separate flights, collect a suitcase, sleep in the city, and fly onward the next morning. Same airport, same destination, entirely different experience.