It is worth being honest here rather than pretending pre-booking wins every time. If you are travelling light, leaving from central London at the middle of the day, heading to a nearby airport with hours of slack in your schedule, an app will usually be quick and may cost less. The case for pre-booking is strongest where the stakes and the uncertainty are highest: early departures, late arrivals, groups, heavy luggage, long-haul flights, unfamiliar airports, and any trip where being late is genuinely expensive.
Put simply, on-demand optimises for convenience right now. Pre-booking optimises for certainty later. Which one you want depends entirely on how much the journey matters.