Well, they will most certainly improve and further our knowledge of the outer limits of our Solar System and interstellar space in general. We already know that interstellar space is nowhere near empty; matter does exist there, but merely as atoms, molecules and elementary particles. Most of which were created as a result of stellar explosions (i.e. supernovae.)

The Voyagers have already provided us with some really interesting information so far; for example, they've shown that our Solar System is squashed, which was discovered just recently. This alone is something we could have never even thought of if it weren't for the Voyagers. Both probes, especially Voyager 1, should make it to interstellar space well before they're out of fuel, according to recent estimates -- sometime in the next decade. We'll definitely get even more information out of them.

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"Science is physics and astronomy." -Me
"There is absolutely no law in physics that prevents time travel." -Dr. Michio Kaku