Triangulating measurement methods
MMM, MTA, and incrementality testing often each give a different answer for how well the same channel performed — triangulating means using all three together instead of trusting whichever number you saw first.
Example
MTA credits paid search with driving a lot of conversions because it's usually the last click before a purchase. MMM shows search's aggregate contribution as much smaller once seasonality and other channels are accounted for. An incrementality test on search shows an even smaller causal lift than either suggests. All three can be 'correct' — they're just measuring different things.
Why marketers care
A customer who's only ever seen one of these numbers tends to treat it as ground truth. Knowing why the three disagree — and being able to explain it plainly — is often more valuable to a customer than any single result.