To some people, a diffuser is just a piece of metal that covers a hole in the ceiling. Occupants don't always understand what the diffuser does; instead, they may see it as the necessary evil that blows too-hot or too-cold air on them hour after hour. They often respond by complaining to the building manager or, worse, covering the diffuser with tape or paper.
Occupant comfort is extremely important to diffuser manufacturers, though. They spend a lot of time and money researching important characteristics such as airflow and throw patterns in order to make sure their products provide occupants with the optimum amount of comfort.