For one, they are the basis of our intervals of time:

1.) The day is the time for the earth to turn on its axis

2.) The month is the time for the moon to orbit the earth (approximately)

3.) The year is the time for the earth to orbit the sun

In addition, they cause the seasons

As the earth orbits the sun, its axis points to a fixed point in space. As a result, we (in the northern hemisphere) are pointed toward the sun in the summer and away in the winter. The seasons are reversed in the southern hemisphere, where they have winter when we have summer and summer when we have winter.

Because they occur with great precision, they were the inspiration for the first thoughts about science in many civilizations.