wpf mvvm confusion

Question

as per my understanding about mvvm is.

there is a model (entity class that also implement inotify...), view (xaml code) and some class as vm (kind of controller whi

Answer 1

why don't we simply consider and enhance code behind...

Code behind is often (always?) the simplest approach...if you're a developer. But MVVM is designed to assist more than just a developer. MVVM is for the database girl and the graphics guy too.
Separating M (for the db) and V (for the artist) and VM (for you) allows each person to work independently of each other. So, for example, you don't have to wait for the graphics guy to make a UI before you can wire up the db. You can all work in parallel (in theory).

Separation of concerns means separate jobs.