Earth rotates because of leftover momentum from the solar nebula that all of the planets and the Sun formed within.
The Solar System, and indeed the Galaxy, were formed by the condensation of a rotating mass of gas.
Each molecule in the cloud had its own momentum, and as they came together, their momentums added up, and needed to be conserved.
Conservation of angular momentum meant that any bodies formed from the gas would themselves be rotating. As frictional and other forces in space are very small, rotating bodies, including the Earth, gradually recuces the rotation speed which is approximately 30 seconds per centuary
