I ran into this problem where I added a 'module.css' file to my module's directory but DNN was not including the necessary markup to include it in the page.  After I incremented the version number for my module definition it started showing up. 

I personally wouldn't consider adding a stylesheet to a module that's in development reason enough to increment the version number, so this seems a little silly to me.

So my questions are:

Why does it need to be incremented so my CSS file gets included?

What does DNN do behind the scenes when the version number is changed?

Under what conditions does the version number need to be changed?  Obviously adding module.css is one of them, but what else?