Barbacoa beef, or pork or lamb or goat, is by definition slowly cooked meat. The word is Mexican, and originally referred to a cooking method by which parts of an animal were wrapped in leaves, and steamed and smoked simultaneously over fire.
The dish came to America from Mexico by way of Texas, and is very much a part of Mexican immigrant and Tex Mex culture, as well as the cuisine of Mexico.
So the notion of making it in a slow cooker is simultaneously radical and obvious. And the resulting meat is flavorful and tender, begging to be stuffed in a soft taco with toppings.
Leftovers might be used in chili, stews, enchiladas, burritos or quesadillas; a couple of cups of shredded, cooked beef in the fridge are a springboard for any number of dinners later in the week. This type of one-two-punch cooking is very rewarding.