Why dose Volvo sometimes replace fully usable.

Not sure what you mean by "fully usable spare parts", but if you mean you try to buy a new part from the dealer and they give you something with a different part number, it's typically because an improvement was made to increase reliability, a change was made to address a common failure, or some other change was made that is significant enough to want to tell it apart from the old version. "Superseded" parts and part numbers are real common.