Catholic exorcist Pedro Barrajon in an interview to signandsight.com-dated way back in 2005 answers this question.
"For our freedom! Evil is linked implicitly to the gift of freedom. God made man free. In choosing whether to ban evil or give the gift of freedom, God opted for freedom. Without the possibility to choose between good or evil, there would be no freedom. That means that God values freedom more than all our sins. Animals are not evil but they are also never free. With freedom, God elevated us above animals."
What do you think? Why does evil exist?