This way you can soak/clean/wash the fan blades without risking damage to the pcb from immersing it in liquids. You can oil it too afterwards if you like.
And yes, this applies to all computer fans of all sizes, however be warned, the smaller computer fans (such as those found on GPU's) are a lot trickier to pull the ring off to remove the fan blades.
Step 1: Remove the restriction ring
Also, some case fans have metal clasps instead of plastic, I'll try to get a picture of one later this week.