What are disadvantages of having an impurity?
Impurities are the UNWANTED substances that are present in the substance.. 
There are Impurities in most of stuff including a lot of chemicals and it's very hard to remove them too
However If impurity exceeds the limit its completely Dangerous..
Like for example :


  Surface water (river or lake)
  Surface water can be exposed to acid rain, pesticide runoff, water runoff, and industrial waste.
  
  Groundwater (private wells or public water supplies)
  Groundwater can be contaminated by disease producing pathogens, careless disposal of household wastes, and leaching from landfills and septic tanks.


which can lead to Nausea
Lung irritation,Vomiting, Dizziness, Skin rashes and even Death

For the Chemistry and Chemical Industry impurities might slow down the reaction or may lead to formation of unwanted bi products , Might affect the rate of Crystallization - Most common issue I think

There are lots of articles online out there here are some for you : Impurity in medicine
Impurity in Electrical cells 
Impurity in water
Effect on Solder machine
you can google and find more