Cutting a raceway leaves a sharp edge inside it, and that edge is waiting for the insulation you are about to pull through.
The damage happens during the pull, inside the raceway, behind a fitting. By the time anything shows, the conductor is installed and the burr that did it is invisible.
A freshly cut end has a ragged lip of metal turned inward. Every conductor you pull past it drags across that lip under tension. Reaming takes the edge off and leaves a smooth bore. The two ends in front of you look almost the same from a distance; the difference is entirely at the edge, which is the only part that touches the conductor.
Check yourself: The raceway is cut and assembled and you have not reamed it. Does it matter, since nothing is damaged yet?
Answer: Yes. A burr left inside a raceway can become hidden insulation damage.
The burr does its damage during the pull, not at rest. Once the conductors are in, the damage is done and it is inside the raceway where nobody will see it.
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Written from the owner's poster for this lesson. Nothing added beyond what the poster states.
Written for Texas apprentices as free study material. The Texas apprentice licence is an application, not an exam (Occupations Code 1305.161), and nothing here counts as apprenticeship hours or continuing-education credit. VoltMark is not affiliated with, endorsed by, or sponsored by NFPA, PSI, TDLR, or any state or local licensing authority. Always follow your employer's safety programme and your supervising electrician.
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