JD/LLM joint degrees for US law students: what's the point?

I see that there are some programs at law schools by which students who are enrolled in a JD program study for an extra semester or two and also get an LLM, so their resumes read, “JD/LLM degree, 2015, X University Law School.”

I can see obtaining an LLM in the following cases:

  1. Someone with a JD from a US law school obtains an LLM from NYU in tax law.
  2. Someone with a JD from a poorly-ranked US law school obtains an LLM from a better-ranked law school.
  3. A foreign lawyer obtains an LLM from a US law school.

Otherwise, I don’t see the point.

What are the benefits of, say, a JD/LLM degree from Duke in “Law and Entrepreneurship”, compared to a regular JD from Duke? The regular JD from Duke should open plenty of doors. If the point is to signal an interest in a specific practice area, why not just join an extracurricular activity in law school that shows that interest?

Not trying to bash law schools, but I’m genuinely curious.