The Earth spins due to its initial angular momentum and the conservation of angular momentum. When the solar system was forming about 4.6 billion years ago, a rotating disk of gas and dust known as the protoplanetary disk collapsed under gravity to form the Sun and the planets, including the Earth. As the material in the disk came closer together, it started to spin faster due to the conservation of angular momentum.
Angular momentum is a property of rotating objects and is defined as the product of an object's moment of inertia (a measure of how its mass is distributed) and its rotational velocity. The protoplanetary disk had some initial angular momentum, and as it contracted and collapsed, this angular momentum was conserved.
As the Earth formed from this collapsing disk, it inherited the angular momentum of the disk. The conservation of angular momentum caused the Earth to begin spinning on its axis. Over time, the gravitational interactions with other objects, such as the Moon and the Sun, have influenced and shaped the Earth's rotation, but the initial angular momentum acquired during its formation remains a primary reason why the Earth continues to spin.
The Earth's rotation on its axis results in the alternation of day and night, the Coriolis effect, and the apparent motion of celestial bodies across the sky.