It promotes tension between states as it creates inequalities with laws and policies. For example, one state may ban gambling to ensure morality while a nearby state allows gambling. There is a conflict between the two as the citizens of the state which outlawed gambling can simply visit the other state to commit what is considered criminal activities in their home state in another state which permits such behavior.
The United States is a common law system, which means law is built on cases that came before it. So a Justice, and any Judge/lawyer, looks to precedent when deciding how to proceed in a case.
