Yes, the ancient civilizations understood the importance of the winter solstice because of agriculture. At 22th December the night is longest and the day is shortest. Ancient civilizations believed that at this night the old sun is dying and new sun is born. A new god. Christianity just took this date, birth of new god, transfer it to birth of Jesus because this day was celebrated long ago.
Ancient civilizations understood the meaning of winter solstice since their religious and social and cultural life were tied to the stars and astronomy. The ancients had a very deep understanding of time in terms of heavenly bodies, (stars, galaxy) very often their agricultural activities like sowing of crops and harvesting were influenced by the solstice.
Ancient civilizations believed in the cycle of reproduction when it came to crops. They knew that when the days got longer, it was getting closer to planting season. They counted the hours of sunlight as a new hope for a successful planting season. More sunlight meant longer days for activities of all kinds, as well as more warmth from the cold.
Yes, I believe ancient civilizations understood the importance of the winter solstice. Many ancient civilizations had a much greater respect for and understanding of nature and natural occurrences. Evidence of their understanding and use of the winter solstice can be found in many remains found around the world. Perhaps our modern cultures should learn from them.