Politicians have been imposing import tariffs for many centuries, for a number of different reasons.

In the 17th and 18th centuries, a theory called "mercantilism" held that the most successful country was the one with the most gold and silver within its borders. Selling stuff to foreigners gives you gold and silver; buying stuff from foreigners costs you gold and silver; so obviously you should sell as much as possible and buy as little as possible across national borders. And one obvious way to do this is to charge a high tariff on anything made abroad that anybody tries to bring into your country.

Mercantilism is (or should have been) obvious nonsense. Possessing gold and silver doesn't improve your life much, unless you trade them for things that actually improve your life like food, clothes, entertainment, etc. -- and as soon as you do so, according to the theory, you've made yourself poorer.

In the 18th, 19th, and 20th centuries, there was a different reason to impose import tariffs: to support the survival and growth of a particular domestic industry. This happened for two main sub-reasons: either a politician has lots of supporters in that industry, and wants to ensure the workers have jobs and the bosses make profits, or an industry is seen as "national security critical", and the government wants to make sure there's adequate domestic production capacity that won't suddenly disappear in an international conflict. (I've been told that even earlier, the Protestant Reformation retained Catholic fish-days in order to keep British fishing fleets in business, thus ensuring a supply of trained sailors and shipwrights who could also serve the navy.) In either case, it gives an advantage to domestic producers in a particular industry compared with their foreign competitors, at the expense of domestic consumers. But it typically takes years for an entrepreneur to buy land, build a factory, hire workers, and start doing business in a new location. As a result, no entrepreneur will undertake such a project without confidence that the tariff will be in place for many years. Which is one reason that the Constitution grants taxing power only to Congress, through the time-consuming process of making and amending laws. Any tariff imposed unilaterally by a President, particularly if it changes every few weeks, cannot possibly have much effect on where industry happens.

In his first term, Trump imposed a bunch of import tariffs, US farmers lost a bunch of foreign markets, and he paid them off with more money than the tariffs brought in, blowing up the budget deficit (even before Covid). In his second term, he's taken it much farther, and it's never been clear why, or what he wants to achieve. As mentioned above, it can't be to encourage companies to set up more production in the US, because that would require long-term predictability, e.g. if the tariffs were enacted into law with bipartisan support.

One possible reason is that he believes in mercantilism, 250 years after Adam Smith eviscerated it in An Inquiry into the Nature and Causes of the Wealth of Nations. He measures his own worth by how much money he has, so it comes naturally to him to do the same for the United States. Selling abroad is "winning", buying abroad is "losing", even buying something that can't be made domestically at all. If the goal is to prevent gold and silver (or rather dollars) from leaving the US, imposing 50% tariff rates on Lesotho and Myanmar and similar poor countries makes perfect sense: they still can't afford to buy anything from us, but at least we can stop buying anything from them.

One of his stated reasons is revenue to the Federal government, and it really does have that effect. Of course, the vast majority of the revenue comes from domestic consumers, so it's really just a way of raising taxes without admitting he's doing so (because Republicans don't do that, y'know). This too doesn't make much difference unless the tariffs are in place for a substantial time.

But given what else we know about Trump's psychology, I think the most likely explanation is "tariffs cause pain to other countries, and thus allow me to extort them in exchange for removing the pain." For this approach, long-term predictability isn't necessary; indeed, chaos and unpredictability are part of the negotiating strategy. This is what he's done with law firms and Universities and media companies: threaten them with something painful (for which he has no legal authority or evidence, but he could still cause a lot of pain in the interim before a court overrules him), then demand obeisance and apologies and White House oversight and protection money in exchange for not applying the pain... yet. The "antisemitism" and "DEI" excuses, the oversight to prevent "anti-conservative bias", even the protection money are secondary to the fundamental goal of putting other powerful institutions into a submissive posture.

In surprisingly many domestic cases, it's working. The heads of major corporations, law firms, and Universities have kowtowed to Trump and literally paid him protection money in hopes that (a) he'll turn his attention to somebody else for now, and ideally (b) he'll come to consider them "on his side" and never threaten them again. (a) is likely, but "for now" may mean only a matter of months. (b) is less likely: even the most vociferous of Trump supporters never know when they'll be thrown under a bus.

It's working less well with other countries. A few countries have reached written trade deals in the past six months, not by cutting their own tariffs on US goods (which were mostly very low already, a result of the US-led free-trade consensus of the past seventy years), but by promising other things, such as investment in US-based companies, which requires that they have dollars to invest, which requires that we run a trade deficit with them! Once a "deal" is vaguely agreed-upon, Trump unilaterally changes the numbers in his own favor and announces the deal as completed, then changes the numbers yet again; you can't negotiate with somebody like that. Some other countries have "come to the table" to negotiate, but it's hard to negotiate when neither side knows what the US wants other than submissiveness, so what's been achieved so far are mostly agreements to continue negotiating. Many countries have had popular nationalistic backlashes against the US: it's become almost impossible to buy or sell US-made consumer products in Canada. And every country that the US antagonizes, China immediately approaches with offers of help and cooperation.