Why to use MVVM patterns with Silverlight and WPF development

To begin with, there are numerous and varied ways in implementing methodologies like MVVM, most of which are used based on personal preference. To a novice trying to educate themselves, this leads to disparity in information. Complicating this factor is that many of the articles or videos that attempt to describe the MVVM pattern also include components of other architectural patterns. This adds unnecessary complexity to understanding the MVVM pattern because of the inconsistencies in the pattern being described.

A great artical by Kim Schmidt. Here is the link