Every brisket, shoulder, and chicken is unique. Thickness, fat content, bone structure, ambient temperature, and pit swings all affect how long a cook takes. Two identical-looking pork butts can finish an hour apart on the same smoker.
A thermometer tells you exactly where the meat is right now, not where a formula predicts it should be. It also keeps you on the safe side of the danger zone (40–140°F), where bacteria multiply rapidly. If you only buy one BBQ tool, make it a reliable instant-read thermometer.