An Onward Ticket is a proof of onward travel means that airlines and governments needs to make sure you are not illegally immigrating to their country, so they need to see proof of a return ticket. Standard travel policies require incoming travellers/tourists to have a return ticket upon arrival in the host country. Presenting only a one-way ticket to immigration officers is risky because there is a huge chance you might be denied entry.
So while you can technically travel on a one-way ticket, immigration also needs some kind of official return ticket confirmation showing that you are leaving the country eventually. They won’t necessarily care where that ticket goes, just as long as it’s out of their country.
Instead of going through the inconvenience (and expensive price of a round trip ticket!) of going back to your country of residence only to leave again just to follow immigration policies, why not purchase a reserved ticket or rent a flight ticket for a more affordable fee?
Onward Tickets is a professionally processed ticket reservation that is useful for travellers who need to provide proof of travel on their itinerary. It can be used to supplement travel documents for immigration and visa applications. You get to save time and money by purchasing an Onward Ticket rather than buying a fully paid two-way ticket that you will likely not use it at all.
Long story short:
Have a peace of mind by owning Onward Tickets as part of your supporting travel documents.