In this world there are two kinds of beings created – divine (daiva) and devilish (āsura). I have spoken in detail about the divine kind. O Partha, hear now about the devilish kind.
To make our life smooth, sublime and successful, we have to know about the world in which we live, move and act. Krishna thus explains how the divine and devilish together constitute the world. Earlier also he has emphasized that opposites coexist in Creation.
To have the necessary inner spiritual strength and harmony, a fair knowledge of divine and demonic traits is needed. Having described the divine characteristics, he now proceeds to detail the opposite ones too, to make his narration full and relevant.
Why should there be evil in the world at all? This is an oft repeated question. Everything in existence and expression is based upon comparison and contrast, which alone enables knowledge to evolve. This essentially gives rise to a variety of contrasts. Krishna details the destructive traits also, so that the seeker will not be disgusted any time seeing the unpleasant.