Hi Yousaf ysaf ,
When you have a Cube, with a Subdivision Surface Modifier on it, you only have 8 Vertices, that you can 'manipulate' in Edit Mode, or Sculpt Mode.
That is why you must Apply the Modifier, to get more Vertices to work with.
Whether you Add the Subdivision Surface Modifier in Object Mode or Sculpt Mode, makes no difference. The exception is, when you Add the Modifier with the shortcut CTRL+(1, 2, 3, 4, or 5). Then, in Sculpt Mode, a Multires Modifier will be Added, instead of a Subdivision Surface Modifier.
The Multires Modifier doesn't need to be Applied.
See Lesson 1 in Chapter 2 of this Course.
Hello Yousaf. Applying the SubD is just to have more geometry. For sculpting you need a lot of verts so you can get the shapes you want. If you don't have enough resolution, then areas that need finer detail wont looks as good. And the second one, yeah you're just over thinking, it makes no difference. Everyone has their own unique way of doing things in Blender. Some of us can't let go of bad habits, muscle memory is too strong.