If evil had a purpose, what it might be?

What is darkness? 
..........Is it just not absence of light?

What is sadness?
..........Is it not the absence of happiness?

What is loneliness?
..........Is it not the absence of a company?

What is poverty?
..........Is it not the absence of richies?

What is evil?
..........Is it not the absence of good?

Does darkness have a purpose? or just a result of lack of light?

So why do you think evil has a purpose? If it could be just the absence of what is right.