I mean, if you know what you are doing because you have made for instance BBQ many times before, especially even on the same equipment, it's not exactly "completely guessing".
People did make good BBQ for many years before digital thermometers existed, and without using meat thermometers! I suspect professional experts barbequing all day probably still don't use thermometers. They are have the same heat source many times, and know how it cooks and know how to judge the size of meat, because they've done it hundreds of times.
But yeah, for those of us who are less expert, a meat thermometer can get us much better results.