What is meant by the term "landmark case"?
A landmark case gets cited as a precedent in subsequent cases.
A law gets passed by legislators; as far as most of us know, that's the end of the story. Actually, it's just one more step and far from the last.
Legislatures make laws, and courts decide whether the new laws pass constitutional muster. One factor that courts consider very strongly is, what did previous courts say about the law? A landmark case sets a precedent that subsequent courts are bound by--unless they set a different precedent.