Class Template Inheritance

[JohnW@Wessex]

I have seen many class template that inherits from a normal base class.
What is the advantages about this design ?

I used such a design for my image classes. The base class contains all of the functionality that is common to all image types, but contains no information about any specific type.
A template derives from this which defines a specific type of image from the template parameters (bit depth, number of planes).
The non-template base class allows me to access the common functionality polymorphically or store references or pointers to images without having to know the specific type.