Separation of impure substances from pure substances is necessary because:

Separation is the process of isolating and removing one or more components from a mixture, thereby increasing the purity of the substance. 

In a mixture, impurities are often present along with the desired substance, and separation techniques help in removing these impurities. 

There are various separation techniques such as filtration, distillation, chromatography, and many more that are used depending on the properties of the mixture and the components present in it. 

These techniques work by exploiting the physical or chemical properties of the components of the mixture to separate them.