It seems that in some way or another, all things in the universe 'spin', or as physicists like to say, carry angular momentum. The Earth spins because the cloud out of which it condensed 'spun', and like a bad cold, the Earth caught some of this spin too. Now, as for why it spins exactly the way it does, with its axis pointed 23.5 degrees to the plane of the solar system, and once every 24 hours or so, is still not known, although I personally do not consider this a big issue in science of itself, any more than attempting to explain why each and every snowflake has the unique shape it does. Nature is inventive.