Why are wheels so important for racing? I hear/read everywhere that if you can spend some extra money on the bike, buy better wheels. can someone explain for me why? 
Is it because they are rotating or are there other comfort factors?
thanks
Wheels and tires are the interface between the bike and the road; and are the parts that take the most stress (wear and tear) along with the bottom bracket. (IMO)


lighter wheels will reduce the rotating mass (rotational inertia); you will need less energy to make the wheels accelerate/decelerate.
higher quality bearings in the hub will reduce friction.
better aerodynamics in the rim and spokes (less energy needed to cut through air) 


IMO, In the end it all matters to weight.

If you are doing serious cycling (i.e. keeping time, strava segments, ... ) it might be a good upgrade; but if you are a little less serious about it, maybe just an expensive christmas gift to self. :-)