Why is there a need to purify matter?

That could be understood in two ways :

1) Usage of purified matter:

Unwanted impurities in matter can cause some problems depending on the use :

- For food : toxicity, bad taste ...

- For metals : can render the metal brittle...

- For chemical reactants : can cause unwanted secondary reactions

...

2) Why matter isn't pure in nature;

Because of entropy.

The more the disorder, the more stable a system is.

To maximize efficiency and to reduce foreign materials from the subject.

That way we get the purest most viable test.