Why work that starts without a measurement cannot be defended
Three months in, the answer to "did it improve?" rests on impression if nothing was recorded at the start. Impression does not defend a budget: work that cannot show what it returned does not reach a second year, even where it genuinely produced a result. A baseline closes that argument up front, because it leaves a reference to compare against.
That the measurement is reproducible matters as much as taking it. A score read only from publicly available files means anyone running it on the same domain gets the same result — the client, the competitor, and the next agency. A reporting method that only means something internally becomes indefensible at precisely the moment it needs defending.