We are creatures of free will. We were created with the ability to chose God or chose evil. We were also incarnated upon the earth to learn, grow and expand through experience. One of the experiences is to learn the power of faith and the power of living His word as guided by our inner compass. If you never try to walk, you cannot fun. If you never make effort, then you will not grow. Faith works the same way. God puts tests out there to help assist in your growth. Some will fail and repeat the cycle, and others will wake up and allow themselves to experience happiness and power of faith. Overcoming challenges and tough times are the core of achieving something great. Being faced with obstacles and still choosing faith is what makes it rewarding.
Evil also provides a contrast when learning lessons. If we never experienced clouds then we may not fully appreciate a sunny day. If we never saw night then we would be desensitized to bright days. Evil shows a contrast to God to allow us to compare. If we never had choices or never saw the darker side, then we would be spoon-fed all the "goodness" and not fully appreciate, understand or earn it. That is why things in the Universe work on balance and the yin-yang theory. There is dark in the light, and light in the dark. We as God's children or here to discern the difference and make our own choice of what path to follow.
Source
Lynn, Psychic Focus