If the Earth had no axial tilt, there would be no seasons. The climate would remain the same for a given location on Earth year-round, and life as commonly known would likely not exist.
According to a professor at New Mexico State University, the Earth has an axial tilt of 23.5 degrees from the vertical. This faces the same way throughout the year and affects how much sunlight the Northern and Southern Hemispheres get at a given point in the year. During summer in the north, Earth's axis is pointed toward the sun, allowing a greater concentration of sunlight per unit area of the Earth's surface. The situation is reversed below the equator. As the year progresses, the hemispheres switch places with the north pointing away from the sun and the south pointing toward it and vice versa. During the spring and the fall, the north and south receive equal amounts of sunlight. At the poles, this effect leads to six months of daylight during the summer and six months of darkness during the winter.
Without the axial tilt, this change would not occur. The Earth would get the same amount of sunlight all the time, and the seasons would not exist.