So some years ago, the probe went into some area between our solar system and interstellar space. Getting info back about that is quite useful in learning more about that area.
I'm not sure if its through it yet, but if it is, then we're getting data back about what's after that as well. Does that data change over time? What if there's yet another transition area and we learn about it?
Having something actually there helps us know if all these theories are right, and the more data we get, the more sure we can be