I copied this from fb

The LYN Hardware Support
If you own a Cooler Master 690 with a side panel, and you're planning to install a high-speed 120MM fan at the side panel, you might want to consider getting yourself a 120MM fan grill which is widely available in Lowyat to ensure that the airflow is not too restricted. However, if you're worried about dust, then keep the mesh grill as that can keep some amount of dust out.


how does have a fan grill helps with the airflow?

From my understanding from that statement, get fan grill and remove mesh grill to improve airflow. OR use the mesh to reduce dust, but result in lower airflow

the last question still need some clarification >>

how does the grill helps with the airflow? i thought naked best

It doesn't. That statement was better airflow when compared to mesh.
The grille is just supposed to protect the fan from being hit/touched and damaged by something AFAIK

to ensure that the airflow is not too restricted. << this implies grille also restrict airflow, but not as much as mesh