Calendars help us keep track of time passing and of structuring our future, making plans and communicating these to others. Without calendars, we would be making scribbled notes and hoping this would liaise with the notes others were making.
Calendars help us to stay organized, to remember important dates including those only relevant to our family like birthdays. It is difficult to maintain any kind of efficient schedule without recourse to calendars. The whole world has found this, for some form of calendar is found wherever people have a civilized society.