Usually it is meant to ward off evil coming from witches or whatnot. Often it is blue. Probably was around long before the first known use in Christian art (The Eye of Providence an eye in a triangle representing the trinity and found in a church built by Charlemagne in the 8th century). Just as Christians adopted someone else's symbol, the Masons adopted the All Seeing Eye as a symbol a thousand years later, in the 18th Century, because it was well-known and established as God's eye. As the Masons also presented it: if they had some different meaning it shouldn't pollute other meanings. It's like letting the Alphabet mob take over the rainbow.