The knowledge that leaves with the contractor
Temporary teams take the learning with them.
Because events are staffed by temporary teams, the accumulated understanding of how this particular event works walks out afterwards. An organisation running an annual event with a different contractor each year rebuilds the same knowledge repeatedly and makes the same mistakes.
The counter is documentation that belongs to the organisation rather than to the supplier: the site plan with annotations, the schedule with what actually happened against it, the supplier list with notes, and the debrief. Contracts should specify that these are handed over.
Continuity of even one or two people between events is worth a great deal and is frequently sacrificed to procurement processes that treat each year as a fresh competitive exercise.
Where continuity is impossible, a proper handover to the incoming team, including a walk of the site with somebody who ran it last time, recovers much of what would otherwise be lost.
The other continuity artefact worth insisting on is the site plan as it was actually built rather than as it was designed. Events always differ from the plan in ways that were sensible on the day, and next year's team working from the original drawing will rediscover every one of those adjustments from scratch.