A developer will use an entire artboard exported PNG as a guide / reference to how the app will look (including pixel layout and spacing between components and interface elements) and if you've included instructions on the artboard, how it might function.
Exporting each element as png, svg or jpg depending on how you've optimised the design will give the developer the assets that can be referenced. Just give each element an appropriate name.
Thanks! I see how to export them as individual production ready files now. I wish you could magically turn them into basic HTML (:
I'd definitely recommend upvoting this story in UserVoice for that functionality: Export whole artboards (or entire project) as HTML – Adobe XD Feedback : Feature Requests & Bugs
Glad you got things sorted out!