Many international students get an LLM because they went to law school in their home countries and obtaining an LLM allows them to practice here. I don't know all the details on that though.
I work in tax law so I know many people get LLM's in that field, although most tell me that it is not necessary. I am still on the fence as to whether I should do it or not, I would really prefer not to. It is my understanding that an LLM in other areas is not really helpful. I think tax LLM's are so popular because tax is such a broad area that can be rather technical. It just shows people you have some specialized knowledge.