Remember getting yelled at by mom when you unplugged the vacuum while it was still on? Why did we do it? Partly because we’re lazy, partly because we wanted to see that “spark”. That “spark” was actually an arc, electricity going through the air to from something energized to something grounded. As fun as it is do, arcs can quickly and easily start fires.
Opportunities for arcs exist whenever an energized wire is exposed. Although this could be due to an installation problem, it could also happen after the fact. It could be an issue of misuse, a cord getting pinched under a door, or twisted too much. But it can also be a simple issue of aging materials. Insulation on aging wiring can actually crystalize and break off, exposing the wire.
How do we prevent potential fires? Arc-fault Current Interrupters. These receptacles watch the power going over the wire. If it sees something that looks like it might be a dangerous arc, it cuts the power.