Unlike modern injectors that are simply swapped out, Cummins PT injectors are mechanically actuated. This means the plunger and cup are two separate parts that wear into each other.
The "Pull": If you install a new injector body into an old, worn counterbore in the head, fuel will leak past the O-rings or the cup will sit too low.
The "Time": PT injectors require timing after installation. A dial indicator is used to measure the height of the plunger at Top Dead Center (TDC). This height controls when the fuel is injected.
Answer: You aren't just sealing fuel; you are setting the exact moment of ignition. If the injector is .010" too low, the engine loses 50 HP.