An outlet in the middle of a run gets wired one of two ways. The first is through the device itself: the wires land on its screws and continue on to the next outlets. The second is with pigtails: the wires are spliced together in a connector, and a short lead runs from the splice to the outlet.

The difference doesn't show right away. It shows years later.

Wired through, the outlet carries the full current for everything downstream on its small screws and internal contacts. A screw loosens over time, the connection gets hot. And when that outlet fails, everything after it goes dead with it — then somebody gets to figure out which of ten outlets caused it.

Pigtailed, the current flows past the device through the splice. The outlet feeds only itself. If it fails or comes out for replacement, the rest of the circuit keeps working.

For the ground wire this is not a preference, it is code: the NEC requires grounding continuity to never depend on a device. Pull an outlet out, and the ground path to everything downstream has to stay intact. The same rule covers the neutral on multiwire circuits.

A pigtail adds a few minutes per box. Fast work skips it for exactly that reason — under the cover plate, nobody can tell. The difference shows up about five years later.