Our solar system consists of 3 classes of objects: rocky planets, gas giants, dwarf bodies. Going to Pluto allows us to study the 3rd class. Scientists think these dwarf objects in the Kuiper Belt are the building blocks of planets but did not have a chance to accumulate into one since our solar system formed and studying objects in that area will give us a more complete understanding of what happened during the early age of our solar system.
I think this mission is especially challenging because it's so far away. Mission planners need to account for the considerable latency involved.