There are basically two reasons.
Last summer we had hosepipe bans in Ireland and the UK. Reservoir levels were dropping due to the unusually warm weather and therefore, to ensure adequate supplies, we were banned from watering gardens using hoses.
So it is a prudent move to minimize what you use today to keep as much water in reserve for a problem tomorrow.
The second reason is energy. It takes a lot of energy to clean water before it gets to you, before you put it back in the sea, and move it around. So the more water we use, the more energy we use. This is also true in the home. If you fill a kettle to the top to make one cup of coffee, then that's a lot of energy you have wasted heating that much water.