From Wikipedia:
A tariff is a tax paid by consumers in a given country imposed by the government of that country or customs territory, or by a supranational union, on imports or exports of goods. Besides being a source of revenue, import duties can also be a form of regulation of foreign trade and policy that taxes foreign products to encourage or safeguard domestic industry.[1] Protective tariffs are among the most widely used instruments of protectionism, along with import quotas and export quotas and other non-tariff barriers to trade. https://en.wikipedia.org/wiki/Tariff
Source of revenue. It could be that Trump and Co see tariffs as a way to get rid of, negate the need for, those awful income taxes they hate and don’t pay if they can avoid it. All in line with his affinity for the nineteenth century. As noted, tariffs are a tax on consumers. A most regressive form.