They found the water evidence via spectrometer readings, so the spectra seemed to identify magnesium perchlorate and chlorate, and sodium perchlorate.
Not exactly what we see in earth's oceans as brine, but, then again, that's Mars Brine I suppose.
A hydrated salt of any type is at least confirmation that hydration was going on...hence the water.
If there is fresh water under the surface, glaciers, etc....as reservoirs....as Alan mentioned, that would really help with colonization efforts.
The brine itself could be harnessed to generate power too, and perchlorates tend to suck water from air.
A subterranean brine seep is more likely as a source, but, of course, there can be many sources...and paths of least resistance.
Given the temperatures on Mars, where these are being found...250º - 300º K, that's quite pleasant at the high end at least.
If there had been a salty sea, as the water evaporated, the salt concentrations (brine) WOULD tend to get progressively concentrated.
After a while, brine would be all that's left if allowed to continue.
The low end of the temperature spectrum would freeze the water and help prevent evaporation at least.
So, new
ELEMENTS are highly unlikely, as we understand how elements are made...but new combinations, or what are normally rare combinations on earth, might be more common given the new conditions on Mars, etc.
There are microorganisms, such as halophiles, that could survive in that environment, so, if there were life in the sea before, its possible that its still there in some form.