Spacefaring races can exchange resources for a net gain in attitude towards each other in the Commerce Partner category.
Normally when you have a trade route between two planets, each side gains an attitude boost toward the other in the Commerce Partner category. Imposing a tariff from this planet to the other planet means that the other planet won't gain those attitude improvements toward this planet, but instead this planet gains a monthly boost to its Economic RCI value. Trade tariffs are one-way and universal from one planet to another (aka they are not per-trade route). So unless the other end also establishes a tariff against this end, the attitude bonus keeps happening over here, AND we get points on the economy scale. If a planet changes hands, then all tariffs are wiped out on that planet.