A good concierge can produce a table at a restaurant that has no availability. The mechanism is a set of standing arrangements with venues, not a hidden booking system.
Venues hold back capacity deliberately
Restaurants and theatres rarely release every seat to public booking. A portion is retained for regulars, for late walk-ins and for relationships the business wants to maintain.
This held inventory is what a concierge draws on. The reservation was never publicly available, so its appearance has nothing to do with a cancellation.
Holding capacity back costs the venue something on quiet nights, but it buys flexibility on busy ones and keeps important relationships supplied.
The relationship is personal, not corporate
Access typically belongs to an individual concierge who has spent years dealing with the same reservations manager, rather than to the hotel that employs them.
When that person moves, much of the access moves with them, which is why the quality of the service at a given property can change without any other visible cause.
It also explains why two hotels of similar standing in the same city can differ sharply in what they are able to arrange.
Reciprocity keeps the arrangement working
The venue benefits because a concierge sends reliable guests who arrive, spend and behave well, which is worth more than filling a table once.
A concierge who sends unsuitable guests or who books and cancels repeatedly loses access, since the arrangement rests entirely on trust rather than contract.
This is why requests are sometimes declined even when the guest is willing to pay. Protecting the relationship matters more than any single booking.
Commission exists but is not the main driver
Some arrangements involve payment to the concierge or the property, particularly for tours, transfers and retail, and disclosure practice varies considerably.
For the most sought-after restaurants and events, though, there is usually no commission at all, because the venue has no need to buy demand.
Understanding which category a recommendation falls into is useful. A hard-to-get reservation is generally genuine; an easy one may be routed for other reasons.
Notice is the constraint that actually binds
Held allocations are limited and are committed in order, so the same request made three weeks ahead and three hours ahead meets very different odds.
Providing the request before arrival, with dates, numbers and alternatives, gives the concierge room to use the arrangement rather than to make an exception.
The service works best as a planning function rather than a rescue one, which is close to the opposite of how most guests use it.