Asklemmy

Personally I think you're deep into logical fallacy territory. Evolutionary side effect vs. major advantage -- these aren't mutually exclusive things. And the second question makes even less sense, to be honest. Are you high?

Having said that, sentience is an emergent property of our complicated biology, and from that perspective and trying to tie this back to your question, it is kind of like a side effect of our brain structure and biochemical processes. But like all "side effects" that persist in populations through time, it almost certainly has evolutionary advantages for sentient species, or at least, it did at one time. Evolution itself is not a omniscient, self-deterministic, or static process, though.

As for correcting errors in vision, it's possible. But it's incredibly unlikely that's the only advantage consciousness and/or sentience provides. Blind people are also able to process and filter complicated sensory information in order to perform substantially complex behaviors. And, conversely, organisms we consider non-sentient are capable of performing some very sophisticated behaviors, presumably completely by instinct.