One purpose of this space mission was to determine if water ice is present just under the planet’s surface. The presence of water of any type is a vital clue to determining if life similar to what we are familiar with could have, did, or still does exist on Mars. In the face of some revealing articles today, it appears that there is ice just under the surface where Phoenix landed.
Even though these robotic missions are extremely expensive ($520 million, not quite as much as as, say, questionable war funding but expensive nonetheless) they provide insights into very fundamental questions about how life takes hold.
This mission has only begun to reveal surprises to us.