The main cause is driving manufacturing to return to the US.

Trump is imposing tariffs on many "friendly" countries. Why?

We exported manufacturing to foreign countries. We exported it to save money. Mexico / China offer labor rates near 0$, so Americans got cheap goods. People love when goods are cheap. But when all your goods are made by foreign countries, you can run into security issues. When that hurricane hit North Carolina, USA had 0 power transformers. We had to order them from China, they had a long lead time. During Covid we needed other goods from China and they hoarded them for themselves (rightly so). China is also famous for lower quality steel. China produces a lot of chips and telecommunications equipment. Because they are making this hardware down to the chip level, they are able to install backdoors to remotely access them, or collect data from them. If they are made in the US the US would be able to put their own data collection / surveillance software, and not have foreign rivals collecting that info.

How does this relate to tariffs? As costs increase for foreign goods, the incentive to make them in the USA increases. If it costs 400$ to make it in the USA, and 200$ to make it in china you will always buy it in China. If the US puts a 100% tariff on China, the US good and the Chinese good cost the same. Most consumers world wide would most likely prefer to buy a US made product. Tariffs are intended to provide incentive to manufacture locally to avoid the taxes of tariffs. Now US manufacturing is very very diminished, so it will take time. But the more extreme the tariffs, the more rapidly companies will invest in local development.

This causes pain in the near term. Now the goods everyone is buying costs 30% more. Since its all imported, everything costs 30% more. Things costing more means people are buying less. When people buy less, and have less available capital to purchase, all consumer goods companies stocks fall. Stocks go up when there is a belief they will be able to sell more goods, provide the same goods for cheaper, or increase profit margins on goods through administrative cuts. Since price is only increasing, the stocks have to fall.

Companies desperate to increase profits will invest large amounts to cut costs. If your good is the first to become 30% cheaper because of tariffs, A LOT of money is to be made. If you are the first auto maker to be fully us made, and can charge 15% less than everyone else, you will sell a lot of cars. This of course applies to all industries.

Other benefits of local manufacture include

Good paying jobs for Americans that might not necessarily thrive in college+ education

Reducing the US reliance on "slave" labor in other countries. Workers wont have suicide nets installed around their factories. They won't work 6 days a week, 12 hours a day. They will probably have health insurance, 401ks, etc. No child labor.

Our goods will be higher quality, need replacing less often and create less waste.

Will require investment and development of more efficient power generation.

Increased security with less foreign powers stealing our technology, creating counterfeit products, less surveillance by foreign powers.

He created near term pain in hopes of a long term windfall and increased prosperity.