In order to stay properly hydrated, you're supposed to drink 1/2 of what your weight is. For example, I'm 170 lbs, so I should be drinking 85 ounces of water a day.
Water is vital for your body to work. We're made up of about 60%-75% water, and with all the sweating we do just to maintain our core body temp, you need to replace it somehow! You could sit in a bathtub all day and soak it in through your cells, but who has time for that? Much easier to drink it.
As for weight loss, drinking water throughout the day not only keeps you full and satisfied, it also cuts down on how much food you eat and flushes out all the crud. It keeps you "regular"
and keeps your organs working in tip top shape.
It's super, super important! I carry around a 20 oz. blender bottle I picked up at GNC. I use it every day all day, and I love it. I've gotten so used to having water around that when I don't, I get incredibly thirsty and crave water.