What ttaskmaster said.
Fan specificcations use open airflow for CFM .. a fan hanging in an open space with absolutely nothing blocking it's airflow. Reality is even a round wire or honeycomb grill restrict 10-30% of a fan's airflow. Obviously the lower it's pressure rating the more the grill lowers the airflow.
Static pressure rating is the maximum pressure the fan is pushing when air stops flowing. 'Static' means stopped, stationary, still, no movement.
While we may use a fan to move air about in really hot weather, I doubt anyone is ever using a fan blocked so it can flow no air .. at least not if they know it's not moving any air.
What does this all mean? Well, we need fans that move air when used with the even more restrictive front grills that block even more area and where most of us also have filters to keep dust out of our systems.
Now you asked again 'what does this all mean?' It means case fans that are similar to fans used on restrictive air coolers or quality radiators make the best case fans.
I will also add to all of this that with modern motherboard fan control abilities many (maybe most) of us are now controlling our case fans the same way CPU and GPU fans are controlled .. fan idle when system load is at idle, and speed up as system load / temperature increases. You know, like CPU coolers and GPU coolers have done for donkey's years .. and notebooks/laptops have done forever.
This also means the fans have much less airflow at lower speeds becuase their abilty to overcome resistance drops dramatically when their speed is lowered.