yes clay bar is neccesary atleast twice a year unless your car is not much in use and is garaged.
clay bar cleans the paint off particles that may not be seen to the naked eye also clay bar does not remove tar completely just removes the head of the tar if you know what i mean.
in the uk the procedure to clean paint is to wash car then use something called tardis to disolve the paint of tar completely then a new thing called de ironizer is used to rid the paint of iron contamination then we would clay bar!
also when i persoanlly clay bar i wash the car and leave it wet and use plenty of quick detailer.
i know its different in pakistan as you can not get much materials out there