Back wired outlets came along as a present to contractors so a home could be quickly wired. They use spring tension on a tang the locks into the wire. In time, arcing can develop there and the outlet needs to be repaired. The repair is to move the wire to a screw. Yes, arcing can be dangerous. Newer code uses arc fault interrupters that prevent this danger. These can easily be retrofitted if you are concerned. When I’m doing individual outlets, I always use the old fashion way of using the screws. Shame on me.