You can technically eat a brisket at 180° Fahrenheit. But this isn't doing justice to the brisket or all your hard work.
When you are slow cooking a brisket, the internal temperature is rising. There is a good amount of fat on the outer layer and inside of a brisket. This is where a lot of the flavor of brisket comes from. If you take the brisket off at 185 there will be a distinct line of the meat and the brisket when it cools down.
If you keep the brisket on until the internal temperature is 204° Fahrenheit, this will allow the fat to melt and render into the meat.