Load forecasting is done to decide which power generating units need to be taken in line in a certain period of time. This is done to determine the best mix of generating units which will give the lowest cost of generation.
The maintenance activities of the various power plants are also planned based on the load forecasting. Investments for new power plants which may be required a few years down the line are also planned based on the load forecasting.
The load forecasting is done using historical data and factors, such as seasonal variations, projected economic activity, population growth, etc