Is consciousness just an illusion?

I understood you to be using the word "purpose" in its use as a synonym for "function", because that's the only way to use the concept that makes sense in a conversation about the nature of consciousness. As in, for instance, the following sentence:

"The purpose of the fusiform face area in the human brain appears to be to categorize and recognize visual data as representing a face."

If you seek a "reason to exist" that lies outside this meaning of "purpose", then the only credible answers to such question have to come from the appropriate sciences, or the question as framed has no legitimate meaning. So a rock, having no parts that serve any functions within it, would have a "reason to exist" that can be discovered through the chemistry of geology. But if the rock were part of a human designed mechanism, like a pulley system, then you could describe it as having a purpose in the first sense.