The reason why planets, such as Earth orbit around the sun is because of the pull of the sun's gravity. And the reason why it travels around the sun is due to Earth's velocity which is in the direction perpendicular to the force of the sun's pull. If we didn't have a sun, the Earth would just travel in a straight line. Imagine if superman were to throw a football with all his strength, it would keep going straight across the Earth which circles the Earth and will eventually hit him, that is basically what we are doing to the sun, we are the football and the earth's velocity is superman's strength. And the strength is hard enough so that gravity doesn't pull the football down to the ground. I am not sure, but I think the reason why we rotate is due to the Earth being at some certain angle.