Presumably the question you're asking is why does a unified self representation require consciousness. (Split brain cases are easy examples of how a break in unification results in incoherent behavior.) The brain nominally performs functions as cascading behavior of atoms whose structural relationships correspond to various functions. But there is no unification at the unconscious/atomistic level therefore a new representational regime is required that can ground the higher level unification.
A successful organism exhibits a high level of competence at reacting appropriately to environmental/sensory states. The "light's being on" is how the brain represents being situated in a world and the significant features therein. Representations within this gestalt are inherently meaningful. For example, phenomenal pain brings with it competence at protecting bodily integrity. The memory of pain becomes part of the explanatory narrative for the monitoring function that tracks progress towards goals ensuring coherent behavior (imagine being fearful of a stove but not knowing why). The contents of consciousness is the semantic engine that induces competent behavior over time on otherwise naive entities.
A successful organism exhibits a high level of competence at reacting appropriately to environmental/sensory states. The "light's being on" is how the brain represents being situated in a world and the significant features therein. Representations within this gestalt are inherently meaningful. For example, phenomenal pain brings with it competence at protecting bodily integrity. The memory of pain becomes part of the explanatory narrative for the monitoring function that tracks progress towards goals ensuring coherent behavior (imagine being fearful of a stove but not knowing why). The contents of consciousness is the semantic engine that induces competent behavior over time on otherwise naive entities.