Why is turbidity an important measure of water quality?
Because turbidity is an indicator of suspended sediment which can negatively affect both the life forms that live in the water as well as those who use that water for consumption.
Turbidity is an indicator of suspended matter or sediment.
By itself, the greater the turbidity, the less the ability of sunlight to percolate to aquatic plants. The suspended sediments can also smother the different forms of aquatic life.
If the suspended sediment contains pathogens and/or toxic chemicals, that will affect aquatic life too.
Turbidity in fresh water also affects those non-aquatic life forms who consume that water.