Why A "DAM " is necessary for a hydroelectric power plant ?

DAM are made to produce hydroelectric power because in DAM the potential head is gained & this potential head converted to the velocity head because for generating extra high power high flow/jet of water is required.

In Dams Potential head energy coveted to the Kinetic energy & by the help of this kinetic energy we gained electrical energy by coupling of hydra turbine to Generator. 

The dam is only necessary to create a difference in height and gather enough water to get a steady flow, so the water falls down and can pass trough a turbine, due to the energy of gravity

To rotate the turbines for power generation, the pressure flow of water is essential and that is the purpose of a dam for hydropower production.

Dam is built to store water back side to make head high level for the water increasing the potential energy to rotate turban and generate electric power

A Dam is a source for storing water built across a River and stores the water upto design levels .

The excess water stored shall be used through Penstocks to run the hydro turbines coupled to electrical Generators. 

Most of Dams are mainly built to restore the water  for the irrigation process. Then the Human thought how to utilize the Water energy to generate the Electricity by Installing Big Mechanical Stage/ Water Turbine to convert the Hydro- power to electrical Power by controlling on the pressure of the water flow to move up to the turbine which is in coupling with Electric Generator to convert the Difference head energy to the kinetic energy and then to electric energy in the stater winding of the Generator which is connected to Power Transformer to step-up the voltage to High Voltage and the to Transmission Line.

Load in a power station is always not same. If there is no dam then the speed of the water turbine can be controlled as the demand required. Moreover during Rainy season huge water can be controlled without dam.

the pressure flow of water is essential to rotate turbines and that is the purpose of a dam for hydropower production

Accoding to me dam is required to store water that can be further ued for generation of energy. which is depend on the hight of Had as high as hight more potential will produce and will converted into kinetic energy.

To store energy in the form of potential energy to convert it into electricity

Dams are built to store the water to create the high head,so the potential energy of water can be converted into kinetic energy to move the turbine of the generator and electrical energy can be generated.