Answer:
It emphasizes the cyclical nature of the year.
Step-by-step explanation:
What the author is conveying through the text is not that each year is exactly the same, but that, while seasons make the year slightly different depending on the period, the things that happen are very similar.
To show this point, the author compares what happens each season with people, with vegetation, and with the day/night cycle. While they change, they are not so different in reality, and are cyclical in nature.