this is all cool and all but I have no idea when is this useful
HI Abdullah JJulaybeeb ,
That's a good question🤔
Although this Course is to show you what specific Modifers do and how to use them, not so much when to use them, I'd also would like to know use cases of the Mask Modifier.
In well over 12 years, I have never used it (or have it seen being used) and actually forgot that it existed😉, but I'm sure that it has its uses.
There's cool uses for it. Lucas in the BaseMesh Course uses it to mask away the part of his characters he doesn't want intersecting with the clothes. Those parts aren't going to be seen anyways and them being opaque would only cause trouble, so he masks them and it works perfectly. Here's the course if you haven't seen it:
https://cgcookie.com/courses/basemesh-create-stylized-characters-quickly-with-blender
It is used for animation mostly. It reduces vertex count during rendering and as Omar points out it prevents clipping(Faces that poke thru other faces). It is commonly used with character that have different outfits. instead of creating multiple versions of the character with each outfit. You just mask out the body part that is hidden by the clothes. Programs like poser and makehuman make extensive use of this. Another less common way it's used, is in isolating rigged components. This is common when you have a very complex model and need to do weight painting on areas that my be blocked by other parts. Like the webbing between fingers and armpits if there are going to be close up shots where this needs to be more accurate. The armature setting is useful for seeing only the part of the mesh that you are currently animating. That is why it doesn't update until you switch back to object mode. this is very helpful for FX and secondary animation. Like the small details that Pixar is famous for.