Water is pretty dense, as you know. Very expensive to import, and you would need a lot of it, even if you recycle every drop. Supplying it locally would be very important.
There was never life on Mars.
Salt is a precipitate from water. And like the guys at NASA said - it is long term. Not just some freak flash flood (melting comet???).
And of course it isn’t proof of life on mars, but it does raise the possibility of sustaining a Mars base sometime in the future and grow crops brought up from earth.