First of all, they provide us with the ability to meaningfully note the passage of time. One week from today will be my daughter's first birthday. The second thing calendars do is notify us of the cyclical nature of our planet. Thanks to a more or less fixed tilt of the earth's axis and a regular period of revolution around our sun, our world gets hot and cold on a predictable schedule, and the patterns of life take note. Flowers bloom in the spring.
Animals hibernate in the winter.
Leaves fall in autumn. We get re-runs in the summer. It's the circle of life. For various reasons primarily relating to food, the planting and harvesting of, we've needed to know when to expect the seasons to come around again. The problem has always been that humans have picked bad ways to note that passage of time.