A brisket that comes off the pit at 09:00 is not finished (sample). Wrapped in butcher paper and left in the warmer for two hours, the juices settle back into the meat and the fat keeps rendering slowly.
Slice it straight off the pit and the juice runs over the board. Slice it after the rest and it stays on the slice, and in your mouth.
The paper matters too: foil steams the bark soft, paper lets it breathe. That is why your tray comes on the same paper.