It's not the traditional BBQ meats (briskets, butts, ribs, chuck,etc.) where this matters. It's with meats that have to be cooked to or past a specific temperature to be safe to eat like chicken, pork loin, chops, etc. Going by feel with these meats is probably fine for the home cook, but if you are cooking to serve to the public and meet health department requirements you need to make sure that things are safe to eat.
Also, as mentioned, if you are doing large cooks, a fast and accurate thermometer is essential since you can get things temped and get the smoker closed again and not lose as much heat.
Also, as mentioned, if you are doing large cooks, a fast and accurate thermometer is essential since you can get things temped and get the smoker closed again and not lose as much heat.