The most exciting finding relates to the history of the body of water that once filled Jezero crater, perhaps 3.5 billion years ago, now long since dried up. Although the existence of such a lake was detected from orbital images acquired a few years ago, Perseverance can now add substantial detail to the crater's history.
On the floor of the crater the rover discovered igneous rocks — produced by crystallizing magmas. These suggest the possibility that the crater was once filled with a lava lake, before the water lake. Sometime later, water flowed into the crater to produce a shallow lake on top of the igneous rocks, creating salty mudflats that were likely good habitats for any martian life that may have existed.
Later, the lake deepened, and we find evidence of a slowly flowing river winding back and forth in much the same way that rivers on Earth do. Finally, there was a period of river flooding that carried very large boulders into the crater.
All of this is important because it tells a story of a Mars very different from today. When the rocks Perseverance is exploring were created billions of years ago, Mars was active — it had volcanoes, lakes, rivers, and habitable environments. But today it is cold, dry, and quiet. Bringing samples back is the best way to understand how early Mars worked, and what happened to so vastly change it.