SPQ-9B is not only part of the gun fire control system. This was only the case for the SPQ-9A.

As you wrote, the SPQ-9B is also used to detect and track low-flying missiles and aircraft. Its better precision, based on shorter wavelengths (X band), complements the S band SPY-1 or SPY-6.

I wonder, why for very different radars similar names are used - both for the SPQ-1 and SPS-73 "variants". In reality, these are different radars.