A process server is instructed to deliver legal documents to a named person or company and then produce evidence that delivery took place. In England and Wales, service must be carried out in the way the relevant procedure rules require — for civil claims, Part 6 of the Civil Procedure Rules — and the server provides a statement or certificate of service recording the date, time, place and manner of service, plus a physical description of the person served where service was personal.
That evidence is the point of the exercise. If service is later disputed, the court looks at what the server did and how it was recorded, so the quality of the proof of service matters as much as the delivery itself.