I think it sometimes depends on your case. In my recent trip to South Korea (Incheon), where I entered from a third country, the immigration officer asked/confirmed me about the details of my trip, including the number of days to stay, the hotel you stay in (I didn't decide at that time), and the purpose of entry. This took me more time than other passengers.
Although I have my onward ticket, however, it seemed that it is not a must-have. And I have frequently heard that people are allowed to enter without having one.
However, the airline forced me to buy an onward ticket much like any airlines usually do; it is because if you were denied to enter the country, it is the airline that has the responsibility of having you back to the departing country.