Mars Pathfinder was first an engineering test
Of course it too did what a lot of failed landers didn't do - returned data
and images. :^) And considering that only two missions before it had
succeeded and that was 21 years earlier, it was a big boost to what
we have now.
If I recall correctly, they now assume MP's landing site was once under
water judging by the position of the rocks.
Exactly how much of Mars do we now think was once under water at one time?