...and lots of it:

With a radar technique, astronomers have penetrated for the first time about 2.5 miles (nearly four kilometers) beneath the south pole’s frozen surface. The data showed that nearly pure water ice lies beneath...

...enough water ice at its south pole to blanket the entire planet in more than 30 feet of water if everything thawed out.

This is important for a couple of reasons. First, it raises the possibility of finding evidence of extraterrestrial life on Mars. Even finding a fossil would be groundbreaking. But maybe a few microbes have survived in some slushy underground pool somewhere. If so, we could be studying extraterrestrial life this century.

Second, water is an essential resource for any self-sustaining Martian outpost. As many have pointed out, we need self-sustaining off-world colonies to insure the long-term survival of humanity.

To be self-sufficient, any human outpost has to have agriculture. Agriculture could provide food, oxygen, maybe some clothing, and the aesthetic appeal of having greenery around. Agriculture requires organic nutrients, CO2, a stable warm temperature, light, and water.

Organic nutrients would have to be imported in initially. After that first generation composting would be essential. We humans can provide CO2 by just breathing. Heat and light could be provided by nuclear reactors.

The other requirement for a self-sustaining colony is the ability to manufacture technology from the surrounding raw materials. This may require nanofactories.

UPDATE: Phil discussed other evidence for water on Mars last December.

UPDATE II: I forgot to mention the other obvious benefits of water on Mars. Water to drink of course, but also, that much H₂O can provide virtually unlimited hydrogen for fuel and oxygen to breathe.