We have our four seasons because of the tilt of the Earth's axis. At different times of the year, the sun's rays hit different parts of the globe more directly.
Without the tilt of the earth's axis, we wouldn't have seasons. Instead, the areas around the equator would receive the most sun and the northern and southern hemispheres would be stuck in a gradual gradient of hot to cold. The seasons would not change, it would be about the same temperature year round.