A ScienceDaily item (here) reports on a study lead by R. Shulman into the energy consumption of consciousness.

…functional magnetic resonance imaging has shown that many areas of the brain, not just one or two, are recruited during tasks such as memory tests and are scant help in studying the state of being conscious. (and) the amount of energy used in such tasks is minute, about one percent of baseline energy available to the brain. “Neuroimaging has been looking at the tip of the iceberg,” Shulman said. “We looked at the rest of the iceberg.” What is the other 99 percent of energy consumption doing? Shulman and colleagues have proposed that it is needed to maintain a person in a state of consciousness. Heavily anesthetized people are known to show approximately 50 percent reductions in cerebral energy consumption.

…Properties of (consciousness), such as the high energy and the delocalized fMRI signals, allow the person to perform the interconnected activities that make up our everyday lives. Shulman suggests that these more energetic properties of the brain support human behavior and should be considered when interpreting the much weaker signals that are typically recorded during fMRI studies.

Nothing that costs that much metabolically is going to be a frill; consciousness must be important if it is that expensive. The function need not be obvious though. For example, consciousness might be a necessary part of forming memories and having memories of events is a very valuable thing.