What should the purpose of a draft of a research paper be?
The draft itself is only the term for something that is surely not finished. The purpose depends on what you intend to do with it.
Drafts are often shared with collaborators to have a common ground for further discussion, fixing some notations and in general keeping them up-to-date with the progress.
Drafts can be just for personal use to keep track of ideas and what has already been done.
Draft is also the version of the paper that is 5 pages over the limit and you know you will need to make changes to fit in.
Draft writing is normally not the goal itself, instead it makes sense to think about what the draft will be used for and then write a document that is useful for this.
It is a good idea to start sketching out the paper before writing it. That way you know what you want to write where. If you want to call that first sketch a draft, then the purpose of that draft should be a "glorified outline". Then you fill it in. If you are very good, or the paper is very simple to write, then you may get it right in one go. Probably, you will have many further drafts, which will (hopefully) gradually become closer to the final product. So, the answer to your question is in one word: both.
The purpose of a draft is to organize your thoughts about what you intend to say and how. You can continue to fix the language and flesh out the details as you work on it, but your draft should make clear what your paper is about and what it has to say on the topic. It should organize your ideas, arguments and evidence in a logical and understandable way.