What is the purpose of using weighing factors for each parameter (temperature, pH, nitrates, total phosphates, and turbidity) when calculating the WQI value?

Various weighting schemes are used to prevent data from being misleading. For example, the most common weighting scheme is an average of values, but this can allow one value to be extremely low, while the rest are extremely high, and the averaging will allow the final result to look acceptable. On the other hand, some weighting systems use squares of deviations from the minimum recommended levels, which greatly increases the impact of a low value. Weighting factors may vary between teams of researchers, but are usually set depending on the history of the area and the methodology being done.