In the BBQ world, mustard is quite often used as a binder to adhere spice rubs to the meat, which are then usually smoked low and slow (as opposed to grilling, which is hot and fast). In my opinion, the mustard doesn't show up much in the flavor because the spices in the rub take over, and there's always bbq sauce, too.
Best gift I ever got myself - oops, I mean my husband 🤣 - was a smoker. Now I spend my summer evenings sitting on the deck, drinking my cocktails and watching him cook.