Saturn: magnificent rings, a planet-sized moon, and dozens of smaller moons. Three spacecraft had flown by before Cassini-Huygens was launched in 1997. But this mission wouldn't just fly by and snap some photos. It was going to get up close and personal.
Jets were discovered on Enceladus in 2005. Enceladus is one of Saturn's icy moons, and the jets are geysers of ice and water vapor. Later study showed that they came from a liquid ocean beneath the icy crust. We know the composition of the jets because Cassini flew through them.
We know there must be an energy source, that there is water, and there are hydrocarbons (organic molecules). This makes Enceladus a promising place to look for life.
We have seasons on Earth because Earth's axis is tilted. Although Jupiter's axis is tilted only 3°, Saturn has a tilt of 27° which is similar to Earth's 23° tilt. Jupiter doesn't have much in the way of seasons, but Saturn certainly does. When Cassini arrived at Saturn, it was winter in the northern hemisphere, the north pole in darkness and the rings tipped up.