To further break down what others are saying about the cost to send water to space, it cost thousands of dollars per kilogram to put something into orbit. Every extra bit of mass you want to send up needs extra fuel. The numbers are a bit all over the place depending on which rocket you use and other factors, but as a rough rule of thumb after googling around a bit, you can kind of figure on it taking about 100kg of fuel to shoot 1kg of payload into space, which means at the low end that 1kg costs around $5,000 to launch. That’s how much it costs to fight gravity and air resistance to get into space.

I’m gonna bounce around between units a bit because I’m American and I think in our crazy units by default. Generally speaking, an adult needs about a gallon of water a day. That’s a bit less than 4 liters, a liter of water weighs about 1kg. So just to get 1 crew members water supply for one day, you’re looking at about $20,000. Multiply that by however many crew members you have and how many days of water you need, and it adds up fast. And that’s before you account for the water you may need for other uses, experiments, equipment, etc. Once it’s up there, space vessels are mostly a closed system, you can recycle the water you have up there to some degree, but there will be some losses, it takes some time and energy to reclaim that water, and you don’t want to be skating by with exactly as much water as you theoretically need, you want to have some in reserve in case it’s needed.

The ISS has a crew of 7 people. Even if we assume our water recycling is efficient enough that the water you used yesterday can all be reprocessed today and be ready to be used again tomorrow without any loss (which I’m sure isn’t the case) you’d still probably want at least 3 days of water per person, so you have at least 1 day of reserve water just in case it’s needed (in reality I’m sure they probably need several days worth of water, if not a week or a month’s worth) so 3 gallons x 7 crew members x $5000 = $105,000 at a theoretical minimum just to send up the ISS crew’s water supply, and again the actual cost is probably several times that much.

And even if water magically had no weight, it still takes up space. For pretty much all practical purposes water is incompressible, you can’t really make a kg of water take up any less space no matter what you do to it. That’s space that could potentially be used to send something else up but can’t because you need to send up the water.

The moon, however, has about 1/6 of the earth’s gravity, and no real atmosphere worth mentioning so no air resistance to fight against, so you need a lot less fuel to get something into space from the moon. So if you have a moon base that’s capable of extracting water on the moon, it could be a hell of a lot cheaper to send that water to wherever else you might need it in space than if you launched it from earth.

Not to mention any other resources we could potentially get on the moon, Mars, asteroids, etc.

And as others have mentioned, we could potentially split that water into hydrogen and oxygen to use as rocket fuel (and breathing oxygen.) Looking longer-term, if we manage to end up with a self-sustaining moon base that’s able to grow their own food, and meet their other needs from the moon’s resources with excess to spare, the moon could essentially become the first pitstop on the way to other planets. We send our astronauts up with just enough supplies to make it to the moon, then they load up with more fuel, oxygen, food, water, etc. and head off to their destination from there, saving us the cost of launching all of those supplies into space. Kind of like if you were to start off on a road trip with a quarter tank of gas because you know just across the state line the gas is a lot cheaper so you’ll fill up there and save a few bucks. We’re probably a long way off from that, but sometimes you have to plan well ahead.