Simple life lives here on Earth in the driest of dry places. Now Mars is dryer still, but that does not preclude the possibility of life still existing there.
Furthermore, this is valuable information for any future manned Mars mission. Any such mission will need a native supply of water. And if there was water on Mars at one point, then there must still be at least a small amount left, though it's probably locked up in hydrates and under the surface.
Finally, information like this is valuable as it shows that water on planets is very common (we've found it on Earth, Venus, Mars, and the moons of Jupiter and Saturn). This lends credence to the idea that water is common on extrasolar planets.