Water is an extremely valuable rescource throughout the galaxy. Most life forms cannot live without it, but other than the obvious fact that water is neccesary for most life, it's also an extremely valuable source of hydrogen and oxygen.
Hydrogen is widely used as fuel, as well as several other purposes. Oxygen can also be used as a fuel, but it is mainly used for breathing.
Water ice is mined from many frozen moons and dead worlds, where it is either purified and shipped off as water, or separated into hydrogen and oxygen.