Diffusers are part of the HVAC system. They provide uniform air throughout the building space.
As air comes through the main duct, the diffuser captures the air and splits this forced air into smaller streams. When there is dust and other pollutants in the HVAC system, diffusers exterior are visibly dirty.. They must be cleaned.
It is no use however to clean diffusers without addressing the main problem lying in the main duct i.e fine dust and other pollutants.
Beware of those that claim that diffuser cleaning is duct cleaning.