How does this metaphor make literally any sense? In what way is type checking similar to making sure you do not kill a person because you can't tell the differe

How does this metaphor make literally any sense? In what way is type checking similar to making sure you do not kill a person because you can't tell the difference between two organs that look completely different and are in completely different parts of the body? Here's a metaphor that makes exactly as much sense: "Construction workers should wear hard hats." "If surgeons needed to wear bulletproof vests to perform surgery, that would be silly!"