Why does earth rotate
There are many theories prevelant among the scientific community about the origin of the earth and the earth's rotation. One of the theory suggests that solar system, initially was a rotating cloud of dust, rocks, and gases. This was a result of the Big Bang. As time passed by, small rocks, due to gravitaional pull, combined to become bigger bodies. Due to their different masses and velocity each of them started to orbit in their own path (orbit).
Earth's Rotation
These eventually became planets. The asteroid belt found between Mars and Jupiter, is believed to be formed because it failed to become a planet.
The earth's rotation is the result of its initial angular momentum and tidal acceleration. As there is negligeble amount of friction in space the Earth is able to more or less maintain its rotation. Although the rate of rotation varies, from time to time, the variation is not large enough to be noticeble by common people.