Some countries, e.g. UK, departure tax increases if you're not in the lowest cabin on the aircraft. If you're in anything other than economy on a multi cabin aircraft you'll pay the higher rate, so if trying to decide between say a Economy, Premium Economy or Business,
Although the Business cabin will have a higher FF mile earning rate than Premium Economy, the tax will be the same as the Premium Economy fare.

(Single class airlines like MaxJet or flights on Concord would pay the lowest rate as they have only one cabin)

Op-ups from Economy to something higher help to avoid this extra tax liability.