For example, i’ve read somewhere that we need around 3,7Litres of water to produce a single cigarette and 2700Litres of water to produce a single t-shirt. Why do we need so much water to produce stuff?
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Water is very useful, very abundant, required for life, and the cheapest thing you can buy. But water isn’t “used up” the way your factoid implies.
Useful: water flows. It can hold enormous amounts of heat, and can carry a wide range of other substances. So if you need to move stuff from point A to point B, whether that’s heat in a power plant or dirt on a floor, putting it in water and moving the water is a great choice.
Abundant: the surface of the Earth is covered in the stuff. Apart from rock, which is inconvenient to move around, water is the most common substance on the planet. There’s hundreds of times more water than air.
Life: Because it’s useful and abundant, when life evolved it selected water as the universal carrier substance for biochemistry. (A lifeform that evolved to use say liquid methane to carry out biochemistry wouldn’t get far on Earth!). This makes it even more useful to humans. Most of the thousands of gallons you mention was used to grow tobacco and cotton.
Cheap: water is literally the cheapest substance you can buy. Water for farming in drought-prone places like California costs a [few cents per ton.](https://aquaoso.com/water-trends/california-agricultural-water-prices/). In those same places, purified drinking water costs a few dollars a ton. Sand and dirt in comparison cost $10-$50 a ton because you can’t carry them in pipes. Even in the desert, water is literally cheaper than dirt.
Finally, I think it’s oversimplifying to the point of being dishonest to talk about water being “used up” the way we use oil or coal. Usually the water is still water when we’re done with it, it’s just slightly modified but still useful. Water that enters a public water supply goes through a human household and ends up contaminated with sewage; after treatment that gets dumped into a river where maybe it gets taken up as cooling water by a power plant, which returns it to the river slightly warmer than it was but otherwise unchanged. Later on it might get used for irrigating crops, where it evaporates and falls again as rain somewhere else.
All of these changes are important and have consequences, but water used for one purpose is still fit for use for other purposes, so it’s really not useful to say “X uses 2000 gallons but Y uses 4000 gallons”. A gallon of sewage is not the same as a gallon of power plant effluent or a gallon of radioactive wastewater.
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