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.
Step 2: Clean the fan
This isn't something I recommend doing a half-ass job on, as you'll only be taking it apart a month later because all the dust you left on the fan will collect even more dust, especially if it's still wet when you start using it again.
I myself soak the fan blades in a secret blend of hydrochloric acid and rubbing alcohol (I'm only joking about the former).