Well, one theory is that Trump wants individual companies to come to him and beg for exemptions to the tariffs. This would consolidate his power: Nobody would dare to do anything that would oppose him, for fear of having their "tariff exemption" revoked.
Another theory is that Trump has some complicated plan to reduce the US's debt; again, it involves using tariffs as a "stick" to push our allies into doing what we want. Summarized here: