If you look at what happens under hallucinogens - most of which act so as to make what is usually unconscious conscious - then it seems as though consciousness is a filter - which selectively eliminates the least important things. Attention is a form of selective consciousness.

Humans function less effectively if their consciousness is continually flooded with sensory inputs that are usually unconscious - since then the important and the unimportant are muddled together.

We are conscious of as little as we are partly because consciousness is a kind of meta-analysis - and is a relatively expensive feature. For systems with limited computational resources, sensory overload can be a big problem - and the filtering done by consciousness represents a large part of the solution.