Airlines answer for flights, hotels for rooms, car rental companies for vehicles. Your traveler lives in the gaps between them, and that's where trips break: one late flight puts the transfer, the check-in and the evening booking at risk, and no one is responsible for all four.
Every extra handoff is a place things get lost. Without a shared layer, a single question, like whether this reservation can be changed without losing the rate, may pass through four or five people before anyone answers. Each handoff adds delay, and each one risks losing context.
Book everything through one DMC? Then it already answers for the whole trip, but it also supplied the service, so it ends up judging its own work. OnSpot stays neutral either way, and it adds the most where no single DMC is covering the whole itinerary.