Water is essential, you can only survive an average of 3 days without it. (some people have survived for longer though.)
Water is important for shipping and trade, because, ships. They go across water. Trade happens. It's hard to fit a lot in airplanes and you have to monitor it's weight slowly. Across land, it's also hard to fit a lot in random trucks driving across what was an ocean. (it would also be much more tedious) It's also one of the cheapest ways.
It is, I guess. There is a high demand for water, and a limited amount as well. It would also be profitable because you can get water easily, or something. (I don't really know for this one but the other two are true so I guess you can't select two so yeah.)
higher altitudes have a lower level of oxygen which in turn, trains your lungs to hold and breathe more oxygen which also helps you cycle, run, etc. longer than usual
