In ancient past, many millenia ago, our ancestors were strongly tied to the earth and its cycles.  The world was mystical and mysterious and events in the night sky and cycles of nature were the stuff of magical stories and beliefs.  People believed  if they pleased the gods and goddesses their lives would be abundant and so they held celebrations and rituals honoring these external sources.  
Imagine the fear first people-- who upon realizing the days were getting shorter and the sun less strong believed the world was coming to an end.  They did not know how they would cope in a world of darkness.  So they made supplications and when the days began to get longer, believed their pleas had been heard and their offerings accepted. 

Some cultures like the Mayan people of Mexico and Central America studied the cosmos and cyclical nature of time and built incredible structures where sun appeared thru a certain window or door the day of the solstices or equinoxes.  The pyramid at Chitzen Itza, Yucatan, Mexico has thousands of people flocking to it during the spring equinox to see the shadow of the feathered serpent god KuKulKan descend from and return to the heavans as the sun rises and sets.  
  
In the more recent  agricultural past, winter season was a time  to allow the earth to regenerate itself, to mend and repair equipment, to plan for the next year and to perhaps work a little less hard. It represented the end of one cycle of the year and the beginning of the next.

 In many parts of the world, people still celebrate Winter Solstice as a new beginning and a chance to connect with nature and appreciate the cycles as we make our way physically, mentally, emotionally and spiritually from darkness into light ,accepting the opportunity to be reborn.