DRD Team

In the modeldefs lump, i have the following code:
I was under the assumption that there does not need to be a sprite counterpart for a model, and that one could simply create sprite names in the modeldef lump. I have found that I do indeed need a sprite with the correct nomenclature in the /sprites directory to make this work. What is even more odd is that when I simply add a sprite named "DUAL" to my /sprites folder, suddenly it works for all frames of the model? Is there any particular reason for this behavior? Why do I need a sprite counterpart to my model?

EDIT: For reference, this is why I want to know

Personally, I just use built in sprites. They will never be seen because GZdoom will replace them with the model. So, I just define my actor using POSS or SPOS or whatever sprites that I know are already there and them map the model to the frames.