Just as soil temperature influences crop emergence, the soil and air temperature influence growth and development of the plant. The growing degree day is an approximation of the influence of temperature on the development of a crop.
Temperature is just one important factor.
Many other factors of the environment are important to crop development, but temperature is a very important factor. Thermal time has become a useful factor to evaluate crop development. Development stages can be anticipated from the calendar. Usually corn reaches tasseling July 15. That would be a calendar model.