Soft materials develop wear more easily. e.g. any slight boogering of the threads of a used fastener could translate into rotational torque that doesn't end up

Soft materials develop wear more easily. e.g. any slight boogering of the threads of a used fastener could translate into rotational torque that doesn't end up being converted to a clamping force. (or in more extreme instances, some fasteners are torque to yield, and change shape after their first use, and must be replaced. But I'll presume that's not what they were talking about here)