None of what follows is an argument for urgency. It is simply what advance notice buys.
The first thing is the arrival itself. A collection planned before you fly has a confirmed meeting point, a chauffeur briefed on where you are going afterwards, and time built in for the ordinary business of getting through a terminal. The same journey arranged on landing has none of that, and it is arranged by someone who has just got off a long flight.
The second is that the vehicle can be matched to the journey rather than to whatever is free. A family of five with a fortnight’s luggage and an executive travelling alone need different cars, and that is a decision best made with the details in front of someone rather than at a kerb.
The third is the shape of the week. Multiple destinations, event schedules, shopping days, a corporate itinerary spread across districts, a day trip outside London — planned together, these are set relative to each other, and a change in one is visible against the rest. Planned separately they are a dozen small decisions taken in isolation, which is where the friction comes from.
The fourth is simply clarity before travel. Knowing what has been arranged, what it costs and what happens if a flight moves is worth a good deal when the trip is being planned from several thousand miles away and nobody in the party has been to the city before.
What advance notice does not buy is a guarantee. We do not hold vehicles or promise availability ahead of a confirmed arrangement, and London has weeks in the year when the city is genuinely committed. Earlier is better for that reason too.