I wonder if there is any difference when i select multiresolution modifier on an object instead of going to [edit mode] choose whole mesh then press [W] subdevide smooth. If yes where both are used for and what reason.
There are a few key differences.
First, the multiresolution modifier is just that, a modifier. That means it's non-destructive, and you can always remove it and get back to where you started. The subdivide smooth tool is destructive, meaning that the only way to undo the operation is with CtrlZ.
When subdividing a cube, the subdivide smooth tool will get a result that is much closer to spherical (although not quite perfect) whereas the mutireolution modifier will not give a perfectly spherical result. The subdivide smooth tool's smoothing also doesn't shrink the model as much as it both rounds areas and raises other areas. This isn't really a huge difference, but they obviously just use slightly different smoothing algorithms.
The main difference though is that the mutiresolution modifier allows you so sculpt on a mesh while still having a less dense base mesh to work with in edit mode. Manually manipulating all of the vertices that it creates is both very difficult and also hard on the computer.
Both tools have their uses, and each one is better for certain things. It's all a matter of what you want to do and the fastest or most enjoyable way of getting the results that you want.
The Multiresolution Modifier (often shortened to “Multires”) gives you the ability to subdivide a mesh similarly to the Subdivision Surface Modifier, but also allows you to edit the new subdivision levels in sculpt mode.
Multiresolution modifier is used when you want to do 3d brush sculpting work. As you sculpt from low resolution and progress to higher resolution the modifier allow you to step up your mesh resolution. Example of such models are cloth, faces, landscapes or fine details.
For the regular subdivide its used for subviding models that does not require sculpting. Often used along with box modeling technique where you start your model with a regular box or single polygon plane. Usually applied on models like cars, planes or any hard surface models.
Hope that answer your question.