A visitor log records who came to a site, when they arrived and left, and who hosted them. It answers three hard questions: who is in the building right now, who was here on a given day, and can we prove it.
The log is only as good as its sign-outs; a book full of arrivals with no departures fails exactly when it matters, at an evacuation roll call or an audit. Retention rules apply too: keep entries only as long as your policy says, and never let one visitor read another’s details.