Most people build trips outward from the outbound flight. They pick a destination, find the cheapest departure date, and then work forward - hotels, activities, internal transport - with the return flight treated as an afterthought. This is backwards, and it quietly causes more itinerary problems than anything else in the planning process.
Return flights on popular routes fill up faster than outbound ones, particularly on long-haul trips where the inbound leg terminates at a major hub. If you’re flying into Southeast Asia on a flexible itinerary and leaving it open-ended, you will eventually face a situation where the only seats available on your preferred return date cost twice what the outbound flight did - or don’t exist at a reasonable time at all. Booking the return first removes that variable entirely.
It Disciplines the Rest of the Planning
There’s a secondary benefit that doesn’t get talked about enough: a confirmed return date forces you to be honest about your itinerary. Travellers routinely over-schedule trips, assuming they’ll have more time and energy than they actually will. When your return is fixed early, you build inward - how many days does this country actually need, where does the buffer sit, what gets cut - rather than building outward and realising too late that you’ve left yourself one night in a city you needed three for.
This matters especially on multi-country trips. The internal logistics of getting from Country A to Country B almost always take longer and cost more than the initial research suggests. A fixed endpoint gives you something to engineer backwards from.
The Practical Part
If you’re booking with points or miles, return availability is often the binding constraint - not the outward journey. Frequent flyers already know to check both legs simultaneously before touching anything. Cash bookers rarely do.
For trips involving long-stay visas or entry requirements tied to proof of onward travel, having the return ticket already issued solves that at the border too. Immigration officers asking for it are less common than travellers fear, but when it comes up, scrambling to buy a refundable placeholder flight on your phone in a visa queue is a bad experience that’s entirely avoidable.
Book the return. Then plan everything else.