Some visa applications, border checks, or airline procedures ask travelers to show a return or onward travel plan. The request usually exists because authorities want to understand whether the traveler has a plausible route, intended dates, and a plan to leave or continue onward.
Buying a final paid airline ticket before a visa decision can be risky. If the visa is delayed, refused, or issued for different dates, the traveler may lose money or need to change the itinerary. A temporary reservation helps separate the administrative proof from the final purchase decision.
The exact requirements always depend on the embassy, consulate, airline, destination, nationality, visa category, and personal situation. That is why a temporary reservation should be treated as a support document, not as a universal rule.