I looked around and found a few facts and examples as to why the Winter Solstice is and has been so important. From my perspective it’s the returning of the light, the start of a new year.
The winter solstice occurs at the instant when the Sun's position in the sky is at its greatest angular distance on the other side of the equatorial plane observer’s hemisphere, the winter solstice as the middle or "extreme" of winter. This system is based on the sun's apparent height above the horizon at noon. This year it is December 21 at 12:04 GMT in the Northern Hemisphere.
Many people can experience mild depression during the first part of winter. Exercise, light therapy can reinvigorate the body from its seasonal lull and relieve winter blues. I can think of quite a few folks that might feel better in the winter if they understood in this.
A few examples of its importance to earlier cultures follow (read this if you have way too much time on your hands):
The solstice itself may have been a special moment of the annual cycle of the year even during Neolithic times. This is attested by physical remains in the layouts of late Neolithic and Bronze Age archaeological sites such as Stonehenge in Britain and New Grange in Ireland. The primary axes of both of these monuments seem to have been carefully aligned on a sight-line pointing to the winter solstice sunrise (New Grange) and the winter solstice sunset (Stonehenge).
The Winter Solstice Festival or The Extreme of Winter is one of the most important festivals celebrated by the Chinese and other East Asians during the dongzhi solar term on or around December 21 when sunshine is weakest and daylight shortest
Goru is the (December) winter solstice ceremony of the Pays Dogon of Mali. It is the last harvest ritual and celebrates the arrival of humanity from the sky god, Amma, via Nommo the Aduno Koro, or the "Ark of the World".
Grianstad an Gheimhridh (Ir tr: winter solstice) is a name sometimes used for hypothetical midwinter rituals or celebrations of the Proto-Celtic tribes, Celts, and late Druids. In Ireland's calendars, the solstices and equinoxes all occur at about midpoint in each season.
Shabe Chelle is celebrated on the eve of the first day of winter in the Persian calendar, which always falls on the solstice.
Soyalangwul is the winter solstice ceremony of the Zuni and the Hopi Indians. It is held on December 21, the shortest day of the year. The main purpose of the ritual is to ceremonially bring the sun back from its long winter slumber. It also marks the beginning of another cycle of the Wheel of the Year.