Why use an MVVM framework for WPF?

Question

Isn't writing the raw code to apply MVVM principles to a WPF project fairly straightforward? Why should I adopt a framework that adds 3rd-party dependencies to my project? Does anyone have any hands-on experience with this? After reading dozens of MVVM articles which all adopt a different approach and/or framework, I'm finding it hard to decice how or even whether to adopt MVVM principles in a new project.