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FTC allows the meat to rest after cooking and lets the juices to evenly redistribute throughout the brisket while continuing to breakdown the connective tissue to gelatin. It takes longer on a large cut, like a brisket, than it does on a steak or chicken.
Also, it gives you some flexibility in the timing of your cook. It will keep warm this way for hours. As you know, briskets will cook differently than an exact looking same brisket. One may take an hour or two longer than the other. So, if you start early enough to ensure that it will be done by dinnertime, then you are flexible if you FTC.
I've FTC'd for 2-3 hours with NO issues and still tons of heat... I did what JMSetzler did, as well as cocoon the meat / foil in beach towels inside the cooler.