What it means
Every platform claims credit for sales it merely witnessed. Incrementality isolates the lift a campaign actually caused, usually by comparing an exposed group against a holdout that saw nothing, or via geo experiments (some regions on, some off).
Worked example
You pause branded search in Kuwait for two weeks while keeping it live in the UAE. If Kuwait revenue barely moves, much of that spend was capturing demand you already had — not creating it.
Why it matters
Incrementality stops you scaling channels that look efficient in the dashboard but add nothing to the business. It is the antidote to attribution theatre.