A set of files says what the last coordinator recorded. It may not explain which promise is still unresolved, why a supplier needs a phone call, or what has changed since the notes were written. An overlapping handover gives the next person access to someone who can answer those questions.
Effective handover combines preparation, exchange, and cross-checking by the incoming person. It is a two-way process, not simply the outgoing person delivering a final speech.
The next person has to use the information
Imagine two festival coordinators working together for a week before one leaves. The incoming coordinator reviews upcoming commitments, contacts key partners, and asks about confusing entries. Trying to use the information exposes gaps that passive reading might leave hidden.
The overlap also needs a clear point at which responsibility changes. Otherwise both may assume the other has replied to a request, or each may issue different instructions. Continuing access to a predecessor is helpful only if it does not leave everyone unsure who now decides.
The cost is time from both people, often when the outgoing person is busy finishing work. That time can be easy to cut because a handover produces fewer visible outputs than normal operations. Its output is continuity: the next person can understand the live situation rather than reconstruct it after the knowledgeable person has gone.