They are strongly attached to the species. For example, your model may contain many properties, but you only need to manipulate a subset of them in a particular view. In this case, you will create a ViewModel to represent the properties that you would like to work with in this view. Thus, you can have multiple ViewModels for a given model, because this model can have many views. The ViewModel may also contain formatted data that is more suitable for display in the view, while the model contains raw data (for example, formatting DateTime, currencies, etc.).
Each time you write one line of C # / VB.NET code in a view, this means that it is a good candidate for a presentation model and / or html helper.