As-built records document what was actually constructed, as distinct from what was designed: final routes of services, actual levels and dimensions, substituted products, and the marked-up or re-drawn information reflecting them.
They are essential to whoever operates, maintains or alters the building next, and increasingly to regulatory records such as the golden thread for higher-risk buildings.
The practical challenge is capture: deviations happen daily and are forgotten weekly. Photographs and messages recorded as work proceeds are the cheapest as-built evidence there is, provided they are filed somewhere searchable.