Earth’s rotation serves an important role in the daily scheme of things.
We all know that the Earth rotates, and that its rotation is what facilitates the regular shifts of day and night. Hence, the Earth’s rotation is detrimental to our survival. However, even though we know that the Earth’s rotation takes place, we do not necessarily know why.
The Earth spins in a counter-clockwise direction, which is why the Sun seems to rise in the East and sets in the West. In fact, every object in our solar system spins, and most of them spin in the same direction of the Earth. This is not just a happy coincidence; there is a reason for it.
According to the law of conservation of angular momentum, if no outside force acts on a moving object, then no change of angular momentum can occur. About 4.54 billion years ago, our Solar System was just beginning to be formed. Instead of the Sun and moon system that we know it to be today, at that time it was just a cloud of hydrogen atoms and dust.
A shockwave, probably from a nearby supernova, scattered the dust and gas, which started revolving. The cloud began to collapse and started to flatten into a giant disk that rotated faster and faster. Eventually, the dust particles started colliding and over the course of a few hundred million years fused together to form the planets, moons, asteroids, and comets, which all continued to move in the similar pattern around the Sun that formed at the center of the cloud. Just as the cloud was rotating as the planets formed, the planets and other objects were also rotating themselves.
As per the law of conservation of angular momentum, since no outside object impeded the rotation of the Earth, it continued to rotate for billions of years. In fact, scientists believe that a large object, perhaps the size of Mars, did smash into the young Earth. However, instead of hindering the rotation of the Earth, it broke off a huge chunk of the planet, which became the Moon, and the impact ultimately sped the planet up faster.
So, in summation the Earth rotates because it was born that way and nothing has stopped it from rotating so far.