You get your nice new Min/Max thermometer and set it up. First afternoon the temperature is 65F when you take you measurement at 4 pm and reset your thermometer. An hour later, a cold front comes through and the next day the high is 30F.
So when you go to take the reading the next day, you record that the high temperature was perhaps 63F, 33 degrees higher than the actual maximum. Only a complete idiot would not immediately recognize that as a problem, so you would start resetting your thermometer at night to prevent it from happening again.
TOBS adjustment assumes that all of the station operators are complete idiots.