A useful way to think about it: on-demand apps are built for spontaneity, and pre-booked services are built for certainty. If your journey has a hard deadline — a flight, a check-in time, an event you cannot be late for — certainty is usually worth more than the flexibility of hailing something in the moment. That is especially true outside normal daytime hours, when fewer on-demand drivers are working and pricing tends to move around more.
Pre-booking also shifts the planning earlier, which suits journeys with more moving parts: luggage, a child seat, a larger group, or a meet-and-greet request inside the terminal. None of that needs deciding at the roadside, because it is confirmed when you book rather than requested from whoever turns up. For a short, unplanned trip across town, that extra structure is not always necessary, which is why plenty of people use both kinds of service for different situations.
If this is a journey you make often, it is worth trying a fixed-fare quote alongside your usual option once or twice, simply to see how the real numbers and timings compare over more than one trip. There is no obligation in getting a quote, so it costs nothing to have both options ready.