Which modern sociological theory states that the best way to analyze society is to identify the purpose that different aspects or phenomena serve in society?

I believe the answer is: Functionalism

According to the functionalism theory, society is complex mechanism that could only operate if the members of society fulfil each of our own function. The results of the work from one member society would be important for the well being of the others.

trade regulation is a field of law, often bracketed with antitrust (as in the phrase “antitrust and trade regulation law”), including government regulation of unfair methods of competition and unfair or deceptive business acts or practices. antitrust law is often considered a subset of trade regulation law. franchise and distribution law, consumer protection law, and advertising law are sometimes considered parts of trade regulation law.

in general the theory of economics says that the more open to trade a country is the better   the result for the same country and the whole economies.

in real life countries impose taxes, quotas, etc which is not so good for the economies.

this kind of commercial war between the usa and china is slowing down the growth of the whole world so we can say that it is a draw back of trade regulations. and the free trade agreements are benefits of trade regulations.

using the scientific method.