It is needed for life. Plants and animals both need to it survive.
1.) More than 70% of our brains are made up of water.
2.) Water is the only reason why we can get nutrients.
3.) The body cannot store water so we have to drink plenty of fresh water every day.
So far, a good example is Mars :
It may have had water at some point but it doesn't now, so along with no water, there is no life as we know it so far. If we find liquid water, most likely we will find life as well...