LLM is often used by foreign educated students to obtain the necessary credentials to take the Bar.
For American educated JDs, the LLM is used to specialize in a particular field of law or to give someone a year to study law in depth to prepare for a career as a professor.
You do not need an LLM to teach at a 4-year college although many professors have it. Glance at the faculty listings of schools - some, maybe most, have LLMs but many do not. Very few have JSDs.
I think most people take an LLM after having some experience. Some schools offer a combined JD/LLM program so you can specialize in a field early on but I do not think that is particularly useful. Better to get the JD, get some experience and then make a decision on whether you want to teach, need more in-depth studying or can get by without the LLM. Most people can get by without it or with normal ongoing self-education.
If you do get an LLM, try to move up in the rankings when you do it. That is, get your JD and then when it is LLM time then move up the ranking ladder - this improves your resume, makes you more attractive for teaching and improves your networking range (never hurts to get another alumni association membership!). Again, take a look at faculty listings, many professors with LLMs have moved up. (Of course, if you are already starting in the top-5 it doesn't matter.)
Bottom line: get the JD and worry about the LLM after you've gotten your feet wet in the real world. Students will want a professor with real-world experience anyway...