Consciousness sounds like an extremely beneficial evolutionary trait to me. Imagine an "unconscious human" who shares the same body as us but the brain thinks it can't think, thinks it is dead, thinks that nothing it senses is alive or real including itself and its own thoughts. That unconscious human can definitively exist as an organism but it would have a hard time to coexist in a community of humans.
Please don't nitpick the usage of the word "think" because I'm referring to "machine style" thinking. A computer can think without being conscious.