The economic buyer can say yes to spending and can end the engagement. They are usually further from the daily work than your champion and care about different things: risk, ROI, and whether this makes their own commitments look good.
Worked example: a deployment can be loved by every user and still be cancelled because the economic buyer never saw a number they could defend in a budget review — which is why the quarterly business review is aimed at them rather than at the users. Gotcha: if nobody on the engagement can name the economic buyer by week three, that is a risk item, not an oversight.