Look at any map. Most political boumdaries were originally set by river courses. If you go to war with a neighboring county, one of the tools you might use against your enemy is the control (or even removal) of their water supply. Building a dam at a headwaters, for example, can completly prevent a nation from capturing any of that water downstream. Diverting watercourses can also prevent your enemies from attaining that water as they had historically. This is a huge issue that can have far reaching impacts on human quality of life, ecosystems, and more.
This is an important question. There is certainly an uneven distribution of water. And if experts are correct, water will become more scarce in the future. With this stated, there are several reasons why there is inequal distribution.
First, the most important reason is due to geography. Some countries are in places in the world where there is little access to water. So, for instance, the Middle East does not have too much access to water; in fact, must of it is the desert. Hence, you would need to transport water, which is costly and time consuming.
Second, wealth also plays an important role. Nations with more money can afford more of everything. Moreover, they have better technology to purify, treat, and explore for water. It is a fact of life that finances place a very large role in any nation's well being.