Change History
The accumulated record of every change made to a site over time, with dates.
Change history is what lets a site accumulate knowledge rather than just accumulating work. When someone asks why a page is structured a certain way, or whether a tactic was already tried, the answer is a record instead of a guess.
This is the thing most often lost when vendors rotate. Each new specialist arrives with no history, repeats work that was already done, and leaves before the next one can learn from it.