Central air conditioner and other industrial ducts needs cleaning for a number of reasons that could be summed up in the following :
post installment cleaning :
after being installed most the times dust and dirt and some plastic wraps or tape might be left in the ducts, with time it will collect more dust, bacteria and mold, causing the whole thing to be less efficient and producing less clean air.
sanitizing and killing molds and bacteria :
an uncleaned duct and ac no matter how clean the external filters are is still vulnerable to bacteria and molds, with time produced gases that the duct is dealing with is going to end you up with these infections, an infected duct will put your health in danger and even your inventory specially food since mold is air born.
insulators have a life span, if exceed black spots would start to show at the air flow vents, that will will travel through the air as black dirt particles causing breathing issues and infections.