The Default shader is automatically applied to every model that appears in the scene. When you apply a shader to an object using the Append or Replace by Type selection from the contextual menu, the model remains.

Edge Cartoon shader A. With the Default shader. B. Without the Default shader.

If there is no shader applied to a selected object, “Default shader” appears in the Shader 1 column of the Shader manager. Use this shader as a quick way to reset shaders applied to your model.

If you want to remove the object so that it reveals only the effect of the shader, right click the Default shader in the Scene browser and select Detach Default shader from Selected Objects.

You cannot delete the Default shader, you can only detach it from all or selected objects.