that helps the rub stick to the brisket. You don't taste the mustard. It's very, very common in Texas brisket-making. In fact, those who do not use mustard are the exception.

Exceptions to mustard are some people who will mix mustard with something else, like pickle juice or hot sauce, and those who use nothing at all.

I personally rub the brisket to create some tackiness with the fat. Seems to bind just fine.