When seasonality is discussed in an economic study, we are referring to the fact that economic variables experience occasional fluctuations or regular changes over time. In this way, these variables become predictable and allows them to be studied temporarily.
Seasonality allows us to know different aspects of the current situation since they have become predictable by the studies of time series. This indicates that, frequently, certain patterns of behavior are formed that allow us to know the evolution of certain economic aspects in a certain period of time. This is the case, for example, of demand for goods or services, supply, consumption, prices, etc ...
The different periods that are succeeded by variability will be called "seasons", hence the name of this phenomenon.
Regarding the importance of measuring seasonality: producers' forecasts can be improved by adapting their workforce and the requirement of materials according to the season in which they are and the moment in the calendar; since the behavior that the demand will have is known.
On the contrary, there is the lack of seasonality, which is the opposite effect. That is, the distortion of the results due to seasonality, obtaining an adverse effect or less than that expected for the time according to the time series in which it is found.
That is why a company or public body cannot base its predictions (especially on demand) exclusively on seasonality.