SpaceX is a company run by Tesla founder Elon Musk. The company's many goals include building a rocket that can get to Mars. The company has already been working toward this goal for some time.
Essentially, the goal of eminent domain laws is simply to say that there are times, even in the United States, where the individual may have to sacrifice something for the greater good. This law allows the government to take private land for use in various public projects.