To explore truly alien new worlds...
Cassini is proof that as wonderful as the Voyagers were in giving us our first serious glimpses of the Saturn system (yes, I know Pioneer 11 was ahead of them all, but it did not have the imaging technology to reveal much), Saturn's first orbiter from Earth was worth every penny and then some.
And even this is just a glimpse compared to what will be seen some day when we orbit and land on those moons of the Ringed Planet.