People do business across state lines.
Because Americans travel, they may be injured in states that are not their domiciles or residences, and those who cause injury do not necessarily do so in their domiciles or residences. The result is that lawsuits can be and are frequently brought in more than one jurisdiction. Also, sometimes a lawsuit filed only in one jurisdiction might be better adjudicated in another. There are doctrines that allow a court in one state to decline to take jurisdiction in favor of the courts of another jurisdiction. A court may decline to take jurisdiction because the forum is inconvenient for the particular litigation, for example.