I think a prime example is the distinction between a patent agent and a patent attorney). Both a patent agent and a patent attorney have to take and pass the patent bar. After taking the patent bar, both can file a patent application with the USPTO on somebody's behalf. But the patent agent cannot give advice to a patent inventor, which, critically, includes advice on whether or not the inventor's patent might be infringing on someone else's patent or vice versa. In addition, the patent agent cannot draft license agreements or other contracts that would help the inventor earn royalties on their invention. And, of course, the patent agent cannot represent an inventor in court if and when their patent is challenged by another inventor. The patent agent can only prosecute patents inside the USPTO, which goes back to the point someone raised earlier about being an "advocate" vs. being an attorney: the patent agent can be an advocate by helping somebody navigate the complex labyrinth of procedures inside the USPTO, but their advocacy ends there. Anything beyond that requires an attorney.
As with the other "advocate" positions raised earlier, one does not need a law degree to be a patent agent. A Bachelor's will do just fine. In other words, you don't need go through the time, energy and expense of law school in order to be an "advocate." If that is your end goal, then you can get started on that path now and skip the J.D.