The person stopping the machine has interrupted output to protect something harder to replace.

Stopping a line is visible and expensive, and whatever it protects is neither. The person who pulls the handle can be measured against lost output immediately, while the injury that did not happen and the part that did not shear leave no trace at all. That asymmetry punishes caution, so it takes some courage to be the one who stops it. Everything that goes right because somebody intervened is invisible by definition.

If someone halts something on your watch, thank them before you ask what it cost. The willingness to stop is fragile, and it disappears quickly in places where stopping is only ever questioned.