What is 'reasonable doubt'? Why was 'reasonable doubt' an important factor in the jury's decision-making?

A classic definition of reasonable doubt is:

"Reasonable doubt is not mere possible doubt. "It is that state of the case which, after the entire comparison and consideration of all the evidence leaves the minds of the jurors in that condition that they cannot say they feel an abiding conviction to a moral certainty of the truth of the charge."

Basically if there is any doubt about the guilt of the defendant than the jury must ere on the side of the defendant. The 8th juror is the first to bring up reasonable doubt.