Its due to the earth has an axial tilt of 23.5 degrees which is an inclined motion, relative to the orbital plane of rotation. This is the angle of obliquity of the earth from its center.
For this reason, the earth has solstice in summer, winter, vernal and equinoxes and owning to the fact that the continents in the north have winter in the Northern hemisphere and continents in the south have summer.
Without this tilt, the earth would have climatic extremes. Day and night time of the earth are also influenced by the same rotational motion.
