The dynamics of fluids are the foundation of the understanding of water movement in streams and in the subsurface. We need to understand this in order to figure out how to measure river discharge, for example. The basic principles also apply to the flow of air, lava, glaciers, and the Earth's mantle.
The air we breathe and the water we drink (and which makes up most of our body mass) are fluids. The motion of air keeps us comfortable in a warm room, and air provides the oxygen we need to sustain life. Similarly, most of our (liquid) body fluids are water-based. And the proper motion of these fluids within our bodies, even down to the cellular level, is essential to good health. It is clear that fluids are completely necessary for the support of carbon-based life forms.