Trade is good for everybody, at least according to economic theory.
While it doesn’t constitute the biggest part of GDP (in 2014, the total U.S. trade volume was only 25 percent of GDP), it has huge effects on the rest of the economy.
Many jobs depend on the export and import sector and some products could not be made without parts being imported from abroad.
Of course, things look different in practice, as trading parties want to gain advantages over the others by using underhanded means. Every country engages in these practices to a certain extent, but China has been known for manipulating its currency as well as subsidizing certain industries, which then dump their products on foreign markets to wipe out competition (solar!).
