Why are they building a dam at hydroelectric power plants?

First of all, the construction of a half-tina since ancient times served as a protection against floods and to tame the water element.
With the development of technologies and the introduction of electrical energy into life, a need arose for the generation of electrical energy with minimal monetary and environmental costs.
Thanks to the study of physical laws, it became clear that when falling from a height, water has a very large kinetic energy, and thus, when the turbine of generators rotates, a large amount of work will be performed, which will lead to a significant increase in electricity generation.