Erie R.R. v. Thompkins and Swift v. Tyson had to do with government attempting to "legislate from the bench" by creating a federal common law except that this "common law" had to do with commercial instruments and crafting a uniform set of rules for them federation-wide.

Forum shopping was the result which attorners obfuscate all of this with the term "diversity jurisdiction".

Erie R.R. v. Thompkins was basically a spin-off. This closed the door to forum shopping, but the federal common law for handling commercial instruments remained.

Prime example of unintended consequences.