Life as we know it depends upon liquid water. I'm not ruling out the possibility of life as we don't know it; there is some speculation that a cryogenic life form using methane rather than water as a solvent might exist on Titan, but certainly if we find liquid water on present day Mars it makes present day life there much more plausible.
I read a very interesting article the other day that suggested that dissolved in water on Mars is a huge amount of nitrates; in part where the missing nitrogen has believed to have gone. On Mars, the majority of the atmosphere is carbon dioxide, there is only a small amount of nitrogen, less than 2%, and almost exactly the same amount of argon. Now on Earth and Venus, there is about 80x as much nitrogen as argon, so the question is where did the nitrogen go; bound up in nitrates appears the likely answer. Large amounts of nitrates would prevent carbon dioxide from dissolving in water explaining why it freezes out at the poles.
If we're ever going to terraform Mars, we're going to need nitrogen, not only as part of the atmosphere but it's a necessary part of life as we know it. Nitrogen is relevant to the existence of liquid water on Mars today, because at the atmospheric pressure present on Mars, water should sublime directly from solid to gas and not exist in liquid form. Pure water that is; but large amounts of dissolved salts or nitrates could alter the freezing and boiling points of water enough for it to exist as a liquid on the surface of Mars for a brief time at least.